Since new, my 08 regularly has this. The temp locks up for the entire day at exactly what it was when started for the first time.
Kind of funny when it is below freezing in the morning and it says "ice possible" in the afternoon when it is 20 degrees above freezing. Of course it work fine when I think about taking it to the dealer.
Odd that other GM cars (including my 14 yr old Buick) have dead accurate temp readings, and aren't even fooled by sitting in the sun or engine heat when stuck in traffic.