We might be a little late off the mark with this as it was published on November 15th 2007 however we thought it was interesting enough to warrant inclusion.
Toyo Tyre USA have felt the need to put out an official customer advisory (TSD-07-001) warning that the R888, R1R and RA1 type rubber may experience cracking when operated in conditions at or below 0c (32f)
They go on to list 'best practice' for temperatures at or below freezing which worryingly includes "Do not operate the car with these tyres, as the tyres may suddenly fail."
News to all us track day enthusiasts I'm sure. We have emailed Toyo Europe to try and get an official comment.