(Quicky-update, new screenshot. The Apps Status Dashboard recently changed, but is still at the same link.)
If you are the one responsible for administering your Google Apps domain, make sure you know about the Google Apps Status Dashboard:
It gives a one-stop-view to see what Google Apps are up (green=yah!) or down (red=boo). Click on any of the little status dots
At the bottom of the screen you can also subscribe to this page as an RSS feed. Very useful!