The iPhone may be a $400 uber-phone, but it can also be hacked and used a music sequencer.
Here, tn800 demos using supercollider and an iPhone to recreate some classic electronic pop:
Here, masayukiakamatsu demos using and iPhone to control Max/MSP:
Here, cappelnord demos a step sequencer for jailbreaked iPod touch, inspired by the Monome 40h Controller:
Another interesting iPhone music experiment is BeatPhone:
Another iPhone electronic music app under development is iPhone Synth, which is currently in alpha.