Here's BtBx, for the iPhone and iPod touch. I think you pronounce it "beatbox". And it's lovely.
There's so much potential for music apps on the iPhone and I'm one of the people who's very keen to try them out. BeatMaker is top of the tree in the App Store right now, but there's a lot about BtBx that appeals to me.
The interface is cartoon-like, but I like it that way. It works and it doesn't crash, and those things matter more to me than aesthetics. The controls might be a bit confusing to anyone who's never used a sequencer before, but there's a brief user manual built-in. The app was built by the same team that created Rhythm for the PSP.
BtBx is cheap and it's simple, and simple is always good in my opinion. Right now there's no way to import custom samples (but that's on the way), nor can you save your patterns or export them elsewhere -- BtBx's greatest drawback at this time. For many people, that will disqualify it immediately.
But: the limitations and drawbacks haven't stopped me playing with it, messing with the patterns to build something interesting, and that's what I like most. The simplicity encourages creativity.
BtBx is a $3.99 download from the App Store.