Yeah, Cocoa is hot and all, but just try getting that nice shiny Cocoa app of yours running on all three of OS X, Linux, and Windows...
If you want to write a cross-platform app, I argue that GTK is still not a very good solution. I wonder how Adobe is progressing with their cross-platform Cocoa-based CS5.
You'd probably be better off with wxWidgets or Qt than GTK.