I heard that, too. I actually just bought my PS3 and GT5 yesterday, set it up last night. Just getting started, bought a used Miata, did the first license set, exited, and saw the car being displayed in its settings for the first time. It was a few minutes before I realized that it was Jingle bells playing! Pretty nice touch.
I just heard it as well. I get the feeling like this has happened to me before??? Does anyone else remember this happening in older GT's? Christmas music playing during the holidays that is?