Good points.
At that altitude (ironically just greater than 200,000 feet) the vehicle is moving through Maximum aerodynamic pressure (Max Q) but the ice chunk probably would not sublimate. Turbulence in the atmosphere would probably not break up the ice chunk, because the turbulence probably wouldn't impart shear forces on the object. I would surmise that fractionation of the object would require sheer forces from the "slipstream" with differing vectors. Toss a piece of paper out of a window at 80 mph and you won't see shredded paper. You'll see a paper flipping and tumbling out of control. Hold the paper on one corner (imparting a opposite force from the force imparted by the turbulence) and you see shredding.
There must have been a larger ice chunk. The piece we see may have been only one part of a larger sheet on the ET.
KSC's PAO has posted better resolution video files.
http://www-pao.ksc.nasa.gov/kscpao/shuttle/countdown/sts107/vidtoc-b.htm
Clip One
http://www-pao.ksc.nasa.gov/kscpao/video/E212.mpg
This is the same angle as on the Florida Today website.
The initial frames are a bit confusing to an untrained person. You can see multiple white objects in a few frames. However, the forward orbiter attachment is between the tumbling white object and the camera. It appears that the object slides between the forward ET-orbiter attachment posts.
http://www-pao.ksc.nasa.gov/kscpao/video/ET208.mpg
This is a view of the stack port side and shows a large chunk of tumbling ice falling off of the area near the ET intertank region (area between the LOX and LH2 tanks).
http://www-pao.ksc.nasa.gov/kscpao/video/ET208Mag.mpg
This is a magnified and digitally enhanced view of the ice chunk appearing between the ET and orbiter.