Zaplin Music’s Triqtraq is a jam sequencer for the iPhone, but it also works fine on the iPad. The introduction video below shows what you can do with it. The app supports the import of your own samples on iTunes, but not (yet?) for AudioCopy/ Paste. The graphics are not optimized for iPad, so keep that in mind.
Triqtraq is now available at the introduction price of € 1.59 ($ 1.99).
A great amount of Video tutorials can be found on the Triqtraq website.