There's a lot of new knowledge to take in with Snow Leopard; the following links are ones that either I've personally found helpful, or I believe contain fairly essential pieces of information for dealing with the new OS. The first four links are the really important ones.
Overall developer release notes for 10.6:
http://developer.apple.com/mac/library/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_6.html
Xcode 3.2 release notes:
http://developer.apple.com/mac/libr...le_ref/doc/uid/TP40001051-DontLinkElementID_1
Cocoa Foundation release notes:
http://developer.apple.com/mac/library/releasenotes/Cocoa/Foundation.html
Cocoa Appkit release notes:
http://developer.apple.com/mac/library/releasenotes/Cocoa/AppKit.html
Concurrency guide (lots of juicy grand central dispatch info):
http://developer.apple.com/mac/libr...ogrammingGuide/Introduction/Introduction.html
64 bit transition guide (old, but more relevant now):
http://developer.apple.com/mac/libr...ocoa64BitGuide/Introduction/Introduction.html
Overall developer release notes for 10.6:
http://developer.apple.com/mac/library/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_6.html
Xcode 3.2 release notes:
http://developer.apple.com/mac/libr...le_ref/doc/uid/TP40001051-DontLinkElementID_1
Cocoa Foundation release notes:
http://developer.apple.com/mac/library/releasenotes/Cocoa/Foundation.html
Cocoa Appkit release notes:
http://developer.apple.com/mac/library/releasenotes/Cocoa/AppKit.html
Concurrency guide (lots of juicy grand central dispatch info):
http://developer.apple.com/mac/libr...ogrammingGuide/Introduction/Introduction.html
64 bit transition guide (old, but more relevant now):
http://developer.apple.com/mac/libr...ocoa64BitGuide/Introduction/Introduction.html