Okay, so this is an arbitrary debate that has gotten on my nerves from some time. I want to first come out to say that both sides of this argument are whiny and oversensitive.
Non-celebrating folk, when somebody wishes you a Merry Christmas, don't get your panties in a bunch. They are trying to be polite. Just simply say "you too" or "happy holidays" and be on your way. Get over the sight of a Christmas tree in public too. It's such a commercial holiday and the tree is practically void of any religious significance.
The people who are opposed to the saying of Happy Holidays, you are even worse. When someone wishes you Happy Holidays, they are not only trying to be polite, but thoughtful in unassuming what you celebrate. After all, this is a holiday season; Thanksgiving, Hanukkah, Christmas, Kwanzaa, the solstice, and New Years all take place at this time. There are many holidays, not all religious, during this six week span. When someone wishes you a Happy Holiday, they are generically including any and all holidays you may or may not celebrating. And get over the religious persecution, because most of you have stood by while your second-most holy holiday has become so commercialized. You've monopolized the holiday season for years and years, so don't be angry when others are merely trying to be included.
I typically just say both unless I'm talking to someone whom I'm unsure what they celebrate.
In short, this isn't a time to be so hostile over the semantics of well wishing. Get over such petty differences and try to enjoy yourselves. It's a time to be happy, or at least try to be.