NoGodBlog (update: link no longer active) recently raised the question of whether hearing "merry Christmas" this time of year was annoying to atheists. The author noted that this statement is almost always said with good intentions but still reflects the assumption that the both parties share Christianity in common. I'm not sure that's the case. At least, when I hear someone say "merry Christmas," I do not necessarily think that he or she believes I am a Christian.
If one construes Christmas as primarily a religious holiday, then I suppose it might make sense that one would be annoyed by the phrase. But what about those who believe that any religious meaning Christmas might have once had was lost a long time ago and that Christmas now represents little more than a retail event? Would someone who believed this have any reason to react negatively to hearing "merry Christmas?" I'm not so sure they would.
Personally, I do not interpret "merry Christmas" as assuming that I am Christian; I interpret it as reflecting the assumption that I celebrate Christmas. I don't. While I do usually give a few simple gifts to a couple people and sometimes send a few holiday cards that do not mention Christmas, this really is the extent of my acknowledgment. And now that I think about it, I don't think I've sent any cards in several years. In that very limited sense, I suppose "merry Christmas" can sometimes annoy me in that it assumes I celebrate a holiday which I do not.
I wonder how "merry Christmas" feels to Jews, Muslims, etc. Not that Christians necessarily care about their feelings, but it does seem reasonable that persons of other religions might have an even stronger negative reaction than I do.