So there's this thing called an iRig - a MIDI to iPod connector. About $70.
Software (SampleTank iPod) for it is free for the demo, and $20 for the full, and something like $40 for every sample in their sound library.
From what I can tell, you cannot swap out individual sounds to customize kits, but if you buy all the drum packs, you have something like 80 preset kits from your iPod.
Anyone have any experience with this?