I noticed this too. My first Xmas playing GT5, so I was pleasantly surprised from the hidden track.
Also, how do you get the decorations to appear in the Arwheiler Street photo location? I never got any through out the day yesterday, yet I see all these photos in the photo stream in that location with decorations. I can't find any info on how they appear.
Doesn't make sense that they didn't appear for me on Christmas.
I believe you have to Gold the first AMG event, which I haven't done yet either. Hope that helps?