Dzsono: An easter egg at Christmas?!
DrakeFox: Not that uncommon to have christmas "easter eggs" in older games when they were fairly cheap to put in and sometimes the devs put it in without anyone noticing.
Never confirmed it, but supposedly if you play Rise of the Triad on christmas you get christmas music in the background and santa hats on some enemies.
I can't recall more christmas secrets, but there should be ample opportunity to find some around now.
I think it's harder to sneak them in these days than it used to be. When you've only got a few people working on the code, it's easier to slip things in without management noticing.
And management tends to frown on having hidden content which people don't pay for.