Not sure what your background is but for most people learning ObjectiveC and Cocoa is much longer than just learning a new toolkit. I have no problem programming in ObjectiveC with Cocoa but may people really do.
I've shipped in Motif, GTK, and Win32, and I know ObjC/Cocoa; of these 4, Cocoa was by far the easiest. Can you point me at some critiques of Cocoa vs., say, QT?
Again, I get that it's cross-platform. But that's not a huge win for the iPhone, which is so idiosyncratic that any good app is going to be one big special case for that platform.