I was looking for an app for the iphone and found this cool beatmaker so you can make beats on the go. Looks like it could be fun. Here is a youtube video I found where it is being used.
http://www.youtube.com/watch?v=_l0x3RnDxbg&feature=related
http://www.youtube.com/watch?v=_l0x3RnDxbg&feature=related