user30591: Let's say that somehow I generate a new address which happens to already be "taken". I realize the chances of address collision are extraordinarily low as per Is it possible to brute force bitcoin address creation in order to steal money?, but let's just say that it happens.
For argument's sake, let's say I land on 1FeexV6bAHb8ybZjqQMjJrcCrHGW9sb6uF (currently carrying a balance of about 80,000 BTC).
If I were to then sign a transfer to another address, would I be doing anything wrong or illegal? I think it'd be immoral, but does it constitute any legal wrongdoing?
I don't think this is quite like finding an ATM card and guessing the PIN, but more like walking into a Swiss bank, guessing a random account number and PIN, and walking out with whatever is in the safety deposit box, right? Is that "stealing" ?

user29298: Could you get away with it? Probably. Laws have not caught up to Bitcoin. It would be better to do the right thing, and just click one button and generate a new private / public key pair, and move along. 
It's like finding money in the street, except you know that the person who lost it is walking back to find it. Can you grab it and run? Yup. Should you grab it and run? Nope.

user30591: Adding my owner answer here thanks to some input from @Wats0ns and @Murch.
IANAL.
Let's say the following happened:

I generate a key pair that collided with an already existing address.
That address happened to have a balance.
I signed a transfer to a new address under my (and only my) control.
In that transfer I put my contact information (and proved after the fact that I had the private key and had done exactly as everyone suspects, crazy I know).

I'd expect:

The other owner to claim they owned the address via a digital signature (the only way you can claim to own an address).
The other owner to show either that they owned the previous incoming addresses or that they traded with others to result in the funding of the address in question.
The other owner to sue me for those coins back, and press criminal charges for theft.
I would use a "Claim of Right or Ownership of Property" defense for the theft charge.
I might lose the civil case and have to return the BTC.

For the theft charge, my argument would effectively be, "I thought I had just as much right to the coins as they did. We both generated a key pair, and the key pair is how you 'own' an address or any BTC the ledger says belongs to that address. I didn't act maliciously in generating the address, I didn't hack a computer and steal the private key, I just happened upon it."
I think that might be a reasonable amount of evidence to show "I thought it was mine" and hopefully that'd keep me out of jail.
I have no idea what a court would decide in the lawsuit as I don't know of any case law that comes close to this. Maybe it's like a bank accidentally adding you as a co-signer to someone's bank account (not heard of any cases involving this)? Or making an invalid deposit to your account (which does have case law, and you have to give the money back IIRC)?
Either way, I'd be willing to bet that the BTC would have to be returned, but it could be a coin flip...

user5406: IANAL.
What you describe is the act of taking something that isn't yours in full knowledge that somebody else invested effort and money to accumulate it. 
Just as with your Swiss bank example: The rightful owner would be losing the funds that you misappropriated. Taking someone's property without their consent is theft.
I feel that this is independent of the way by which you gained access to the property, but rather derived from the intent of your action.
Perhaps more ambiguous would be scenarios where somebody receives a transaction to a key that you also control, or when you stumble on a key that has a Bitcoin balance that hasn't been moved for decades and could reasonably assumed to have been lost by the rightful owner.

30591: <p>Let's say that somehow I generate a new address which happens to already be "taken". I realize the chances of address collision are extraordinarily low as per <a href="https://bitcoin.stackexchange.com/questions/22/is-it-possible-to-brute-force-bitcoin-address-creation-in-order-to-steal-money">Is it possible to brute force bitcoin address creation in order to steal money?</a>, but let's just say that it happens.</p>

<p>For argument's sake, let's say I land on <a href="https://blockchain.info/address/1FeexV6bAHb8ybZjqQMjJrcCrHGW9sb6uF" rel="nofollow noreferrer">1FeexV6bAHb8ybZjqQMjJrcCrHGW9sb6uF</a> (currently carrying a balance of about 80,000 BTC).</p>

<p>If I were to then sign a transfer to another address, would I be doing anything wrong or illegal? I think it'd be immoral, but does it constitute any legal wrongdoing?</p>

<p>I don't think this is quite like finding an ATM card and guessing the PIN, but more like walking into a Swiss bank, guessing a random account number and PIN, and walking out with whatever is in the safety deposit box, right? Is that "stealing" ?</p>
 29298: <p>Could you get away with it? Probably. Laws have not caught up to Bitcoin. It would be better to do the right thing, and just click one button and generate a new private / public key pair, and move along. </p>

<p>It's like finding money in the street, except you know that the person who lost it is walking back to find it. Can you grab it and run? Yup. Should you grab it and run? Nope.</p>
 30591: <p>Adding my owner answer here thanks to some input from @Wats0ns and @Murch.</p>

<p><strong>IANAL.</strong></p>

<p>Let's say the following happened:</p>

<ol>
<li>I generate a key pair that collided with an already existing address.</li>
<li>That address happened to have a balance.</li>
<li>I signed a transfer to a new address under my (and only my) control.</li>
<li>In that transfer I put my contact information (and proved after the fact that I had the private key and had done exactly as everyone suspects, crazy I know).</li>
</ol>

<p>I'd expect:</p>

<ol>
<li>The other owner to claim they owned the address via a digital signature (the only way you can claim to own an address).</li>
<li>The other owner to show either that they owned the previous incoming addresses or that they traded with others to result in the funding of the address in question.</li>
<li>The other owner to sue me for those coins back, and press criminal charges for theft.</li>
<li>I would use a "Claim of Right or Ownership of Property" defense for the theft charge.</li>
<li>I might lose the civil case and have to return the BTC.</li>
</ol>

<p>For the theft charge, my argument would effectively be, "I thought I had just as much right to the coins as they did. We both generated a key pair, and the key pair is how you 'own' an address or any BTC the ledger says belongs to that address. I didn't act maliciously in generating the address, I didn't hack a computer and steal the private key, I just happened upon it."</p>

<p>I think that might be a reasonable amount of evidence to show "I thought it was mine" and hopefully that'd keep me out of jail.</p>

<p>I have no idea what a court would decide in the lawsuit as I don't know of any case law that comes close to this. Maybe it's like a bank accidentally adding you as a co-signer to someone's bank account (not heard of any cases involving this)? Or making an invalid deposit to your account (which does have case law, and you have to give the money back IIRC)?</p>

<p>Either way, I'd be willing to bet that the BTC would have to be returned, but it could be a coin flip...</p>
 5406: <p>IANAL.</p>

<p>What you describe is the act of taking something that isn't yours in full knowledge that somebody else invested effort and money to accumulate it. </p>

<p>Just as with your Swiss bank example: The rightful owner would be losing the funds that you misappropriated. Taking someone's property without their consent is theft.</p>

<p>I feel that this is independent of the way by which you gained access to the property, but rather derived from the intent of your action.</p>

<p>Perhaps more ambiguous would be scenarios where somebody receives a transaction to a key that you also control, or when you stumble on a key that has a Bitcoin balance that hasn't been moved for decades and could reasonably assumed to have been lost by the rightful owner.</p>
