Last Wednesday I attended a presentation in Austin Community College's on-going First Wednesday Video Game Seminar series on Mac game development. Their focus was Objective-C and Cocoa and (obliquely) iPhone game development. It was also fairly focused on what they were doing, and had done with Mac game development, rather than the breadth of development happening in the Mac game market.
I was kind of disappointed, though, because they didn't even mention Carbon, and were dismissive of ported games. Years ago I got to do Cocoa (and, previously, OPENSTEP) programming in Objective-C, and I really liked it. For a desktop application, nothing beats Apple's Interface Builder for designing interfaces and tying that interface to your code. But for games... Interface Builder doesn't really have anything to offer, and Cocoa doesn't have a lot to offer either. For cross-platform games, Cocoa has zilch to offer, because it dictates language choice. Indeed, even Apple documentation indicates Carbon is likely better suited to the task.
I really enjoyed the presentation, and I'm a fan of Cocoa, but I felt like everyone already working on a non-Mac game was left out in the cold by it. On the other hand, if you're interested in writing a game from scratch, and you want to target Mac users... Cocoa is definitely a great way to do it.
For games, you're probably better off using whatever toolkits are out there that can target OpenGL. Then, presumably, you'd only have to do enough Cocoa and IB in order to make an ogl window display, and you'd do everything else in that context.
As for why at least Adobe is trying to switch to Cocoa... I can think of a few reasons off the top of my head.