After the scrubbed launch on Sunday, NASA engineers walked around the Shuttle to inspect it and found a rather substantial crack in the insulation on the external fuel tank. They also found a piece of foam on the launch structure that apparently fell from or near the crack. It's unclear how big of a deal this is, but it's all over the web: CNN, MSNBC, SpaceFlightNow, and others. The Orlando Sentinel has a story with an image that might be of the crack (they don't say exactly):
No doubt there will be a public statement from NASA about this very soon, as they were planning on trying to launch again on Tuesday. I'll post as soon as I hear anything.