HyperTunes is an interesting looking iPhone app. It is a MIDI sequencer, but not for driving external MIDI sound sources. Instead it is sort of a sketch pad for arrangements, which will drive an internal General MIDI sampler. And I do mean arrangements, there is no way to actually input any notes at this time, so it is just for shifting around bits of MIDI into new patterns. This can then be exported as MIDI or even HTML files.
The developer, Exploded Views Media, brought it to my attention and I was fairly impressed with their demo video. It isn't for everyone, but if you want to try rearranging your MIDI compositions on your phone this looks like a slick way to do it.
You can find more information and videos on their site here.