Since we're still a site in beta, I don't believe it's appropriate for the moderators to make an "executive" decision on whether or not Academia.SE should participate in this promotion without first seeking input from the board.
The basics of the "Winter Bash" are that completing various activities on the board will lead to the awarding of "hats," which can be "worn" by the gravatars. This promotion would last for about two weeks at the end of the year. Individuals can choose to opt out at any time by clicking an "I hate hats" button that will appear on all participating sites.
If Academia.SE chooses not to participate, then this would not affect the accounts on other sites, and vice versa.
To create a "poll" mechanism, I'll add two comments. Upvote according to your preference, and leave a comment or answer below to provide more feedback. We need to provide an answer by Nov. 28, so we'll "close" the poll no later than Nov. 26.