You know, if I was going to make a gut call on this given what I've heard so far, I would speculate that a main gear door popped off over California. I can't think of anything else big enough to give a visible trail in the sky as it came loose, although that's just my impression of how it works. I can't be certain even a small tile wouldn't leave an impressive trail. Look at how small most meteors are. One the size of a grain of sand leaves a visible streak.

The problem with the gear door theory is that I would think a hole that size would wreck the Shuttle pretty damn quick. As it was, the orbiter continued on for about six minutes until it was over Texas.