Like Eamon, I have no problem being wished a Merry Christmas, though I tend to respond with "Happy Holidays". I have often seen my (secular Jewish) father offer a hearty "Seasons Greetings", accompanied by a warm handshake.
Here are some other suggestions I have run across lately (most of which are thought-provoking, even if unlikely to catch on as social memes):
"Merry Xmas, Ymas, and Zmas" (our family will be celebrating Ymas this year)
"Happy Solstice" (at the risk of being mistaken for a pagan)
and, of course, the American Humanists' "be good for goodness' sake"