If you happen to have followed WMATA on Twitter, then you've probably seen how many updates they send out every day saying there are delays.
That doesn't help much when you don't actively have your nose buried in your phone, following only Twitter. What CAN help is the new (and best of all...FREE) iCommute DC app.
It gives you real time, GPS-tracked arrival predictions for Metro buses. You can download it from the iTunes store here.