The Menu Extra API is private. Apple doesn't want developers to use it. That said, it's EXTREMELY useful -- the suggested alternative, NSStatusItem, requires that your app be running at all times and isn't nearly as flexible.

The quickest way to get started is to use class-dump to extract a header file for the framework that contains the API, which is found at /System/Library/PrivateFrameworks/SystemUIPlugin.framework/Versions/Current/SystemUIPlugin.

The class you'll want to extend is NSMenuExtra (and NSMenuExtraView, if you want a fancier menu header than the standard text string).

Yeah, I saw Menu Extra Enabler, which looks like a pretty sweet piece of work. I may just have to upgrade to 10.2 and grab their SDK.