SunVox is a modular sequencing application that has been ported to almost every platform under the sun (sorry, terrible pun). It's especially targeted to mobile platforms - in addition to working on Windows, Mac and Linux, it also works on PalmOS, Windows Mobile, and now the iPhone. I spent a little time with the iPhone version and am pretty pleased with what I've seen so far.
The interface is split into 3 areas which can be resized more or less at will by tapping on the resizer bar. This is very important because SunVox packs a lot of features onto one screen. The top area features a Tracker style pattern editor. Users of Buzz, Psycle, Traktor, etc. will find themselves right at home. The bottom right features a module view where you can create and link up modules and the left area shows the settings for the selected module. Figuring out how stuff works is fairly easy and for the most part everything works as expected. The tracker pattern area takes a little getting used to since you don't move the cursor by direct tapping, but instead by dragging. This setup makes sense when you don't want to obscure what you are tapping on. Plus you can do the pinch/zoom gesture to zoom in and out.