Well, if I know someone is a Jew, for instance, it might be a nice gesture to specifically wish them a happy Hanukah. Or <insert holidays from other religions here.>
Otherwise, nope. People just need to get the f*ck over themselves! If someone wishes you a merry Christmas, and it is obviously coming from a good place, then why the hell should you be offended?! That's just dumb.
However: I might be willing to make an exception in cases where a Christian KNOWS someone is an atheist, and goes out of their way to obnoxiously and condescendingly wish them a "merry CHRISTmas, because it's all about CHRIST, remember?!" In which case their intentions would be anything but good.. And if I were still an atheist, that would p*ss me off, too.
So yeah... It is basically all about the sentiment behind it, in my view.
Someone who's not a Christian, but who still appreciates it when people wish her a merry Christmas, and who really does not mind the word "Christmas" one bit. :)