jbonedev I've seen a lot of sound and music type iPhone apps that claim to let you create or "be creative", but most of the sequencers and drum machine apps out there tend to use loops that you can't really modify or sampled riffs from guitars that you can't change.

If you know how to play a guitar, FretPicker will allow you to play any chord (currently at the top of the neck only.) store chords, play individual notes and any chord changes or chord progressions you want.

EZSoundTracker lets you create beat sequences, change tempo, and structure your sequences into complete songs.