So before deciding to do ClearCoin, I was thinking I was going to clone one or more of the PayPal shopping cart / payment APIs.
The advantage would that all of the existing shopping cart interfaces that know how to talk to PayPal would "just work" with bitcoins (the PayPal APIs can already deal with multiple currencies) -- just replace the https://api.paypal.com/ URL (or whatever it is) and specify amounts in BTC instead of USD or EUR.
The short-term business model would be an E-Wallet for businesses (like PayPal). The medium/long-term business plan would be to get acquired by PayPal when PayPal decided they need to support bitcoin.
If you don't like PayPal, then replace "PayPal" by "Google Checkout" or whatever other, popular payment processing gateway API is already being used by lots of websites.
I still think this is a good idea, but I'm busy, so I'm throwing it out here and hoping somebody decides to do it.