Blizzard claims that they are around but players are not looking hard enough. I don't necessarily believe them due to the prevalence of the halloween decorations a couple of months ago.

Then again, I would not put it past them to implement the seasonal decorations as little easter eggs worldwide...like the occasional Treant in Ashenvale walking around with ornaments and lights or something. And wouldn't it be hilarious to summon Ragnaros and have him appear in a big santa claus suit? Then when he summons his sons, they could all be goblins dressed like keebler elves....