I find it sadly amusing how controversial the Christmas holiday has become as people and organizations attempt to be politically correct. Wal-Mart's banning of "Christams" last year resulted in such a backlash that some groups tried pinning the same thing on other companies they didn't like, claiming that this or that company was firing employees for saying "Merry Christmas" (an untrue story featured on some Fox News shows among other places without them bothering with minor journalistic details like fact checking).
People on both sides get too worked up. If a company uses Happy Holidays, it's not that big of a deal. I had a customer give me grief because the sign on our door said "Holiday Hours" in stead of "Christmans Hours" until I pointed out that the sign included Thanksgiving, Christmas Eve, Christmas Day, New Years Eve and New Years Day, which meant that it covered multiple holidays and not just Christams, ergo the Holiday Hours.
On the flip side, I really don't think many people are going to get offended if you wish them Merry Christmas this time of year, even if they celebrate a different holiday. Any holiday greeting is merely well-intentioned well-wishing, so take it (and give it) for its intent.
Compound that with the fact that Christmas has pretty much become a secular holiday. While many Christians celebrate religious aspects, many folks celebrate it without the religious aspects. If any thing, the ancient church PR plan was too good... it co-opted a popular existing holiday season with their religious holiday (despite the fact that Jesus was *supposedly* born in July), and increased the popularity to the point that the holiday took on a life of its own outside of any religious trappings. Now it's Christmas trees and Santa Claus.
But it's all good. It's a time for family and friends. So whatever, however you celebrate, have a happy one.