Two new mockup/wireframe design apps are now available for the iPad that are worth taking a look at:
My initial impression is that iMockups is the better of the two at the moment… but since I just started to play with them I will reserve judgement for a few weeks.
In general I find that doing a wireframe mockup on the iPad is a more fluid experience then using computer-based tools… it is going to be interesting to see how apps like this develop over time.