Early Speculation

From Instapundit -

"I'M WATCHING THE NEWS CONFERENCE, and it looks like a zipper effect followed by burnthrough and structural damage, leading to the loss of the left wing. They're reporting anomalous heat sensor readings, loss of tire pressure in the main gear on that side, and so on.

"The shuttle can tolerate the loss of a tile or two. But when the integrity of the tile cover is breached, tiles can be pulled off one after another -- hence the term "zipper effect." Then enough heat can penetrate through in sufficient quantity to destroy or weaken what's underneath. This is a well-understood possibility, so expect a quick resolution (by the standards of these kinds of things) if the evidence continues to point this way."

Sounds reasonable.