Axure has offered a powerful prototyping tool for the Windows platform for a number of years now. After listening to their customers, Axure has decided to create a Mac version.
Given the history of the Mac development platform and Cocoa, it's surprising there aren't more user interface prototyping tools for the Mac. It will be enlightening to see if Axure uses Cocoa as it's development framework.
If they do use Cocoa, they may be able to leverage other frameworks — Core Data springs to mind — to increase the fidelity of the models.