iMockups looks great. What it really has me thinking about are business and productivity applications for the iPad that could make the iPad a killer-device outside of books, games, video, etc.
I'm already looking forward to the applications The Omni Group  has planned, since I live and die by OmniFocus on the Mac and iPhone, and OmniGraffle could be useful to me in meetings, sketching up system diagrams, software architecture, etc.  

Basically, I'm saying that I'm sick of writing stuff down with a pen.  I'm left handed and I'm tired of ink stains.  Bring it. Down with paper!