According to a thread on Apple's support forum, there may be an issue going around with FaceTime on the iPad that freezes up the iPad 2's front camera. Users are saying that sometimes, when FaceTime loads up, the front camera will freeze on a frame and not send any more video. Fortunately, the fix seems to be simply restarting the iPad, but obviously, there's something weird going on -- the issue often pops up even after a restart.
The FaceTime for Mac app was just updated yesterday with a few performance improvements, so there's likely an update pending for iPad as well. Hopefully, whenever that's released, it'll fix whatever issues users have been having.