This is being very closely examined. The picture appears to show that the failure happened right above the repair that was made back in 1988 when the plane was built and delamination was discovered on one of the vertical stabilizer attach points.

My local paper had a story this morning (can't find it online) on the accident investigation. It appears that the FDR only recorded a 0.1g lateral acceleration when the Airbus hit the 747's wake turbulence. That's a mild acceleration, far too weak to break off the tail. The FDR shows that the lateral acceleration increased sharply not too long afterwards when the vertical stabilizer broke off.

The article had another interesting piece of info that I hadn't seen before. It seems that when the plane was parked outside at one point during construction, it was damaged during a windstorm. They can't say whether this had any influence on the accident or not.

There are hundreds, perhaps thousands of airliners around the world using composite structures. There are many of them with metal doublers for repairs like the one shown in the photo. This is the first time one of them had a failure - truly a freak accident. Some people are concerned that the long term effects of the repair might've transfered the loads improperly to the other attach points. There's also concernt about how well the repair ages, although there doesn't seem to be any indication of failure in the repair shown in the photo.

My guess is that the loads were improperly transferred to the other attach points. This might've been a failure just waiting to happen, kind of like the C-141 wing failure shown in this link. All they were doing was refueling the plane while it sat on the ground when the wing collapsed. Spooky, and a damned good thing it didn't happen in flight.

NOTE: let me know if you can't get to that link.