Here's an awesome sound creation tool that works for free on the web, or you can download for $.99 for iPhone and iPad.
You can chose the scale and tempo as you setup the generative items. There are 40 scales to choose from.
I picked up my Native American Flute in the key of E and jammed along, it was great.
I downloaded to my iPhone 3 and there isn't enough processing power, it sounds horrible. I am hoping that it runs better on one of the newer iPhones. Please post a comment if you've verified this.
Otomata for iPhone
Earslap website for Otomata