Realizing Your Idea Isn't Original
pedalpete: Wow, you've got a ton of great and positive advice here. 
I agree with almost all of it, but would add this...There are no shortage of ideas, and no shortage of businesses to build if you have the ideas. If the company/market you are looking at has a good implementation with market penetration and barriers to entry, and if you can't SIGNIFICANTLY make a difference, then maybe keep looking. 
Take a read of Rick Segal's current post (and my comment there). 
http://ricksegal.typepad.com/pmv/2008/11/features-of-the-wor...So, your final sentence says it all... go develop, you'll learn a ton going through that process and maybe find more ways to differentiate and improve on the market.Don't forget to let us know when you launch.