
WJLA is currently reporting 1 fatality and 9 injuries. Washington Post doesn't have any more details either.
I received this text message alert from AlertDC at 5:45:

Metro reports massive injuries at this time. The green line and the red line are
affected. Further information to follow.
As someone who rides Metro regularly, I didn't think this type of accident was possible. Very scary.
Update 6/23/2009:
The casualty count is now 9 deaths and 76 injuries. The trains did not collide head-on, but instead one train ran into the rear of the first stopped train.