In a similar thread, a new Mac OS X native application called Antetype just started beta testing. From their site:
"ANTETYPE is a brand new, ground-breaking design application that supports user interface designers at every step of their work. Design beautiful interfaces, invent new widgets, style them and put it all together in an interactive prototype!"
I haven't had much time to kick the tires yet, so I'll hold my thoughts for now. That said, it's great to see an application in this field that was built for the mac first, instead of a Windows port or cross-platform Air application.