We knew they increased the thread pool limit from 25 threads/cpu to 250 threads/cpu, but we also found they changed the heuristics for creating threads to avoid creating a bunch of threads in a short period of time. In previous versions of .NET, threads were allocated at a rate of 1 every 500ms assuming there were queued items. This is documented behavior in MSDN.
In .NET 3.5, it appears to have been changed to a sliding scale where they slow down thread creation as more demand is placed on the pool. This is a change even from the beta and must have been slipped in at the last minute.
Michael documented the whole change on his blog here.. recommended reading.