If you have missing menubar icons (or "menulets"), and previous hints aren't working for you, try typing killall SystemUIServer into Terminal.app. This will force-quit the SystemUIServer process, which handles menulets. SystemUIServer will then automatically relaunch and rebuild the menu, recovering any missing icons you had previously activated.
I made myself a tiny AppleScript app (do shell script "killall SystemUIServer"), so all I need to do now is launch that and wait a few seconds. The only downside to this I've noticed is that the icons often reappear in random order, which I then have to Command-drag back into the order I prefer.
It works for me, and now I don't have to go digging thru System Prefs, or the Core Services folder (or run to Activity Monitor or TinkerTool) to reactivate my missing menulets.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060203133729554