I'm Christian, but I guess I don't have any strong feelings about Merry Christmas vs. Happy Holidays vs. Season's Greetings. If I know someone isn't Christian, or I'm not sure, I tend to use one of the latter two. If I know some is Christian, I'll use the first. But if someone wished me a Happy Hannukah or a Blessed Kwanzaa (or whatever the appropriate greeting would be), I'd be okay with that. I figure whatever they're wishing me, they mean well, and it's not really a serious attempt to convert me or disapprove of my faith, though I recognize it's probably easier for me to feel that way, since my faith's holiday is much more obviously everywhere this time of year than theirs.
Really, I've often used cards with "Peace" or "Peace on Earth" as the sentiment. I can't imagine who would disapprove of that, regardless of their faith or even if they haven't one at all. And because my Christmas card for the last dozen years has featured a photo of my dog with a letter written by him, I have people who don't celebrate Christmas, but who like dogs, who put the card up every year anyway (and he's gotten responses from dogs and cats in Jewish households, though, oddly, none of the pets who live with my Christian friends/relatives have responded).