When my wife asked me why she could not buzz in Yahoo Messenger for Mac OSX, I thought at first it was a one time happening since I also noticed before that there were occasions wherein I could not buzz. When I tried it out, I was mystified. I mean, it cannot happen just like that all of a sudden.
So I decided to check out the menu. Lo and behold, I was surprised that the Send Buzz functionality does not have any shortcut listed there anymore. You can still buzz. But you have to go to the menu and click on the Send Buzz menu item in order for you to buzz. The Apple-G does not work anymore.
Then I remembered that I just updated my Yahoo Messenger for Mac OSX (ver. 3.0.1 build 35554). For unknown reasons, I have no idea why they took out the shortcut for the buzz functionality. If you have wondered why you cannot buzz anymore using the shortcut, wonder no more …
EDIT: For the solution/fix, check the 1st page of comments. Big thanks to @srg. Oh yeah, for Mac OSX Lion, instead of Keyboard Shortcuts, go to Application Shortcuts.