user516: A comment on this answer pointed out that the total amount of bitcoins in circulation could appear to go above the limit of 21m if wallet services use a fractional reserve system. However, in that scenario you actually no longer have bitcoins - instead, you have some kind of bitcoin-credits that rely on a specific 3rd party to be used.
If my understanding is correct, it should be possible to determine through the blockchain whether a given e-wallet service:

Definitely isn't using fractional reserves for a given account, or
Might be using fractional reserves

How would this be done? Has anyone summarised this information for a selection of e-wallet services?

user575: Verifying that an eWallet doesn't do fractional reserve requires collaboration on its part (and its users), and I hope in the future most will collaborate this way.
Every user will choose a unique identifier (which can be unrelated to any other identifying detail if they wish, or not if they want their ownership to be known publicly), and the eWallet will have a public list of the balance of each identifier, rounded to improve anonymity. From this the total amount deposited can be calculated, and each user can verify that his own record is on the list to make sure he's not underreporting. Once in a while the eWallet will move all coins in a single transaction to address(es) provably his own, this way he can show he has all deposited coins under his control.

user153: There are two types of EWallets, hosted (shared) EWallets and hybrid EWallets.  
With hosted EWallets, all the coins are shared (all customers funds are combined) in the host's EWallet.  Examples of this are Paytunia.com, Instawallet.org, EasyWallet.org, WalletBit.com,  as well as the online wallets with the exchanges, such as MtGox.com, Bitfloor, etc.
Hybrid EWallets are those that operate locally either from the browser (using Javascript) such as how Blockchain.info/wallet and StrongCoin.com operate or from a client like how Electrum (Windows, Mac, Linux, Android) or BitcoinSpinner (Android) operate.
With a hybrid EWallet, the private keys are kept locally but transactions route through the EWallet provider.  To protect from loss, the browser-based hybrid wallets store an encrypted copy of the data from the wallet online with the EWallet provider.  The provider doesn't have the password to decrypt the wallet stored with them and thus has no potential to spend your coins stored with them.
So your risk of fractional reserve exists only with the hosted EWallet providers.
One method these providers could prove they are not a fractional reserve would be to submit to sporadic financial audits.  None do this today.
Another option would be for the EWallet provider to treat each account as if it were a standalone wallet.  That way the blockchain would verify that the coins received have not been spent.  No EWallets operate this way either.
When multisignature becomes functional, this will likely be a security feature available from one or more EWallet providers.

516: <p>A comment on <a href="https://bitcoin.stackexchange.com/a/4163/516">this answer</a> pointed out that the total amount of bitcoins in circulation could appear to go above the limit of 21m if wallet services use a fractional reserve system. However, in that scenario you actually no longer have bitcoins - instead, you have some kind of bitcoin-credits that rely on a specific 3rd party to be used.</p>

<p>If my understanding is correct, it should be possible to determine through the blockchain whether a given e-wallet service:</p>

<ol>
<li>Definitely isn't using fractional reserves for a given account, or</li>
<li>Might be using fractional reserves</li>
</ol>

<p>How would this be done? Has anyone summarised this information for a selection of e-wallet services?</p>
 575: <p>Verifying that an eWallet doesn't do fractional reserve requires collaboration on its part (and its users), and I hope in the future most will collaborate this way.</p>

<p>Every user will choose a unique identifier (which can be unrelated to any other identifying detail if they wish, or not if they want their ownership to be known publicly), and the eWallet will have a public list of the balance of each identifier, rounded to improve anonymity. From this the total amount deposited can be calculated, and each user can verify that his own record is on the list to make sure he's not underreporting. Once in a while the eWallet will move all coins in a single transaction to address(es) provably his own, this way he can show he has all deposited coins under his control.</p>
 153: <p>There are two types of EWallets, hosted (shared) EWallets and hybrid EWallets.  </p>

<p>With hosted EWallets, all the coins are shared (all customers funds are combined) in the host's EWallet.  Examples of this are <a href="http://Paytunia.com">Paytunia.com</a>, <a href="http://Instawallet.org">Instawallet.org</a>, <a href="http://EasyWallet.org">EasyWallet.org</a>, <a href="http://WalletBit.com">WalletBit.com</a>,  as well as the online wallets with the exchanges, such as <a href="http://MtGox.com">MtGox.com</a>, <a href="http://BitFloor.com">Bitfloor</a>, etc.</p>

<p>Hybrid EWallets are those that operate locally either from the browser (using Javascript) such as how <a href="http://Blockchain.info/wallet">Blockchain.info/wallet</a> and <a href="http://StrongCoin.com">StrongCoin.com</a> operate or from a client like how <a href="http://ecdsa.org/electrum">Electrum</a> (Windows, Mac, Linux, Android) or <a href="http://market.android.com/details?id=com.miracleas.bitcoin_spinner">BitcoinSpinner</a> (Android) operate.</p>

<p>With a hybrid EWallet, the private keys are kept locally but transactions route through the EWallet provider.  To protect from loss, the browser-based hybrid wallets store an encrypted copy of the data from the wallet online with the EWallet provider.  The provider doesn't have the password to decrypt the wallet stored with them and thus has no potential to spend your coins stored with them.</p>

<p>So your risk of fractional reserve exists only with the hosted EWallet providers.</p>

<p>One method these providers could prove they are not a fractional reserve would be to submit to sporadic financial audits.  None do this today.</p>

<p>Another option would be for the EWallet provider to treat each account as if it were a standalone wallet.  That way the blockchain would verify that the coins received have not been spent.  No EWallets operate this way either.</p>

<p>When multisignature becomes functional, this will likely be a security feature available from one or more EWallet providers.</p>
