I usually say Happy Holidays, but only because I'm talking about both Christmas and New Years. My cards even say "Merry Christmas" and "Happy New Year" on them, then we wrote "Happy Holidays".
I'm not the kind of person who has friends that are easily offended by such things, or we wouldn't be friends for long. I'm fairly offensive on a regular basis and saying Merry Christmas to someone who doesn't celebrate it would really be a fairly minor offense in the grand scheme of things.
When I used to actually have a Jewish friend, I always wrote Happy Haunikah on their card, and they always wrote Merry Christmas on the card they sent to us. I don't think either of us would have been offended if we didn't though, it's the spirit of the greeting that counts. I just like to know I've been thought of.
Nexa