Yeah, I had the same idea (give or take) except I couldn't figure out how to make money on it. If that's not your goal, then the idea makes for an excellent open source project.

Also, you might want to have a look at http://dialog.gs - a beautifully designed flop that is. Guy was using the same position as you - small, portable, disruptive, fits between the chat and the email, etc. It appeared to have flopped because he couldn't get people on it, which is something to think about.