There are so many cool music making and recording apps for the iPad that it's hard to keep up with them all. Here is a recent one that is a 4-track MIDI sequencer called the Nord Beat CoreMIDI Step Sequencer. If you don't know what sequencer does - well, it's sort of like the drum machines of old (though the sounds a sequencer uses don't have to be drums). There is a tempo and time signature you can assign to a box (in this case, an iPad app). Then there are buttons that correspond to up to 16 beats in one "sequence." You can assign a sound - usually a drum hit - to each button to create a pattern over the sequence. Then you can build songs by stitching together a bunch of sequences.
Anyway, here is some info on the Nord Beat: http://en.audiofanzine.com/virtual-instrument-iphone-ipod-touch-ipad/clavia/nord-beat/news/a.play,n.14074.html