I think Jews would probaby feel more comfortable sending Hanukkah cards.
Actually, I do give Christmas cards to my Christian friends, and I make a habit of telling everyone I see during the Christmas season "Merry Christmas," since it's a safe assumption that most people you see in public will be Christian. And being Jewish, if someone says "Merry Christmas" to me, I smile and give them a big, "Thank you! And Merry Christmas to you too!" I can't imagine why people would get upset when someone wishes them a Merry Christmas, especially a stranger who would never know that you're NOT Christian. Unless they're just looking for something to get upset over.
My Christian friends who know I'm Jewish do wish me a Happy Hanukah.
Mark
You're my kind of person! My best friends are jewish (I'm a convert at heart if not in form) and they do not get offended when people wish them Merry Christmas...it's a way of showing appreciation to people and not a curse!