The debug menu in AddressBook is no secret, it's been covered here before. If you don't have it enabled, quit Address Book and open Terminal, then paste this command:
defaults write com.apple.AddressBook ABShowDebugMenu -bool true
Anyway, there are some new things in 10.6. Some items have just been reworked, but it's still worth checking out. The first option in the menu is Show Debug Panel and this gives you a little inspector window called Ye Old Debug Settings. They probably won't mean much to you, but it's funny to see!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090906111727225