I just picked up my brand new IS 250 5 days ago and today I noticed this creaking as well. Mine, however, seems to be coming from under the dash on the driver's side.
I don't know if temperature has anything to do with it (I know it shouldn't) , but it's been under 20 degrees here for the past 2 days. So, I'm going to wait a day or so and see if the noise continues. Right now I'm bummed though - my first "high-end" car and didn't think I'd have to deal with something like this. We'll see...