There are loads of great resources out there for learning Cocoa for Mac development but because Xcode 4 is so new you aren't going to find much aside from Apple's own documentation.
It's still in development and has just been updated to cover Xcode 4. I've been reading the previous edition and it's brilliant: great writing and the first half of the book is applicable to both iOS and desktop Cocoa development. As a relatively new Mac developer (1-2 years) this really consolidated my knowledge of Apple's frameworks and Objective-C.
Thanks so much for your help. I'll definitely check out the book. For others who may be looking, I found these excellent Xcode4 screencasts: http://pragmaticstudio.com/screencasts
The mechanics of using XCode 4 are somewhat different, but all the fundamentals are identical. As far as documentation goes, Apple's online docs are a surprisingly good starting point.
