http://www.inkmesh.com is another ebook price comparison site. I had the same idea a few months ago, but couldn't really think of any way to improve upon inkmesh and ebookprice. Plus, I've found that Amazon, BN, and Borders usually have the same price on popular books. I think Amazon sets the price and the others match it.
I created it to learn Ruby, Sinatra, JQuery, and web development in general. It was a fun little project.