Spaceflightnow is carrying this nice summary of recent Columbia investigation developments. In short: the breach area has been moved a little further down the wing, and now the leading suspect is a "T-seal" instead of a carrier panel. Also, mockups of the various materials and panels are scheduled to be tested by shooting chunks of foam at them out of a nitrogen cannon. Of interest is the use of panels from Enterprise, the original flight-test prototype, in these mockups.