Check out qTweeter for the iPhone
I rarely buy an iPhone app. However, qTweeter - available only to those of you with a jailbroken iPhone - is worth its salt. It enables a quick update to Twitter and Facebook from wherever you are in your iPhone. You don't have to leave the app you're currently using!
In that sense, it's much like QuickSMS (which I consider another must-have app (also requires a jailbreak).
Here's a video of the app at work: