I did a similar thing earlier this year. However, I took a top-down approach: I picked a music style, did a several simple preset musical patterns, and defined a set of rules how to mutate them to something different. There's no system to pick the good mutations though, so it's not really evolutionary, but at least it sounds somewhat musical. The app is already available as online applet: http://www.rinki.net/pekka/acid/ ...

I just developed an algorithmic composition applet, very similar to the first application by Harmonix. The users can control the music dynamically with the mouse. I thought it was unique idea, but these guys did it already 15 years ago!

I think their earlier ideas were much cooler than the Rock Band franchise, too bad they couldn't sell them.