Lately, it seems TC-Helicon have been devoting quite a bit of attention to
promoting Voice Looping (their VoiceLive Touch product, for instance).
Now,
they've come out with VoiceJam for the iPhone/iPad:
http://www.tc-helicon.com/products/voicejam/
Haven't tried it yet (my credit card and the iTunes store are fighting with
each other again). But it seems pretty spiffy for only $1.99.
--m.