We first saw ads for this shortly before Thanksgiving. It was in heavy rotation on Thanksgiving itself -- we saw it many times while at Jen's parents. The Snuggie is basically a fleece blanket with arms, one size fits all. The infomercial has plenty of images of whole families sitting around in Snuggies, looking very much like a cult.
USA Today has a piece on it: Snuggie Gets A Warm Embrace From Pop Culture
I should order one for Jen (which means ordering one for me too, since you get 'one for free'!)