The Shuttle has been flying for 20 years or thereabouts. Do you really think that the tile issue is new? I wouldn't be surprised if the design has always allowed for a few tiles to come off. I bet it has happened all the time. The Columbia explosion was an anomaly.
There are several layers to the heat-shield, the tiles are just the top layer. The reason things ended up the way they did was superheated air got inside the fuselage (an undercarriage bay, wasn't it?)
Now, those filler strips they worked on today are near the nosewheel, hence the concern. But it really isn't anything to worry about.
EDIT:
See the black object? THat's a picture of a bit of thermal tile taken in 1986 from the window of a Shuttle... They landed just fine.
That seems to be the NASA "position", but one of the re-entry photos of Colombia that NASA first tried to brush off as camera shake, showed the shuttle's straight track was joined by another wiggly track immediately before the shuttle broke up. So why, if the camera was shaking was the shuttle's track straight?
There is increasing suspicion that positive lightning may have been the culprit, and that positive lightning gives rise to a much higher voltage gradient than conventional lightning, and so is more destructive.
A bit ironic really, as I think this shuttle flight was to specifically gather information on lightning. Could it be some power is trying to stop us knowing too much? Over to B.O.B.....................