Thanks, G Man.
I called Lexus of Toledo today and am scheduled for after the holidays. The service guy seemed to know very well what was happening. He said "when it's 90 degrees there is no problem but when we get down about 10 or 20, there are ...." I'll need to leave the car for a day or two and there is a loaner so I'll report back.
He also implied that Lexus knew about this so I'm sure that there will be warranty coverage.
Jan 5, 2009 Update: I took the car in to Lexus of Toledo today and the service writer said there are two service bulletins out. Evidently, there is a lot of plastic in the dash and when things get cold, the plastic shrinks and becomes a little brittle and does not make a solid contact with the windshield which may be one of them. I'm guessing the other is when the radio or nav stack as well as the glove box bump the underside of the dash on a hard bump.
We'll see what happens.
Jan 8, 2009; The car is back and the rattle appears to be fixed although I did not have a chance to really take some back rods as a test. It was a warranty repair with an nice RX as the loaner (my request to try one) and I saved a lot of money because my wife's 8 year old MDX is still better than this new Lexus. Funny how stuff works out.
December 2009 Update: It has turned cold here again and in a couple of recent drives, there was just a few little creaks and noise in the dash. Much better but I really want a very quiet car (the LS400, ES300, and an Acura TL that I have owned over the last 15 years were like tombs inside). My lease is up in June and I'm thinking that the new TL AWD looks very, very good.