Last flight of the shuttle?

This is a sad day for NASA. After what had looked like a perfect launch on Tuesday, inspection of the launch pictures on Wednesday showed a falling chunk of foam almost as big as the one that doomed Columbia. No, this one didn't hit, so the crew is presumably safe, despite a couple of what appear to be the commonplace (though not necessarily safe!) kind of dings on thermal protection tiles. But the fleet is once again indefinitely grounded until they figure it out.
Think about it: They just spent 2 1/2 years working specifically to solve this exact problem. They failed. How long will it take next time? Is it possible?
And if this ends up taking a long time once again -- and frankly, it's hard to imagine why it wouldn't -- we may have just seen the last launch of a shuttle.
The fleet is supposed to retire anyway in five years. If half that is eaten up with further redesign work, would it even make sense to continue? Would it still be able to carry out enough missions to justify its continuation? Or, I suppose more likely, would they retreat from the 2010 retirement date?
I'm sure no such decisions will be made for a long time, but NASA is in for perhaps its toughest year yet. And it has had some very tough years.