Aziz Poonawalla, City of Brass
My approach is to always return a "Merry Christmas" with a "Merry Christmas", and I always lead with "Merry Christmas" to someone I know is a practicing Christian. It's the same as someone wishing me (obviously Muslim, visually) Eid Mubarak - a nice gesture of respect. But for the general public or an unknown, I will use Happy Holidays, to be as inclusive as possible. I think that the spirit of the holiday season, which spans Christmas and New Year's, has transcended the religious and become not secular per se, but rather multidisciplinary, and speaks to humanistic values that all religions, atheists, and agnostics share. The divine aspect of Christmas is one facet of the gem, but we can choose any facet to look through, or appreciate it as a whole.
Anyone who chooses to be offended by how someone greets someone else during this season is someone who has missed the point.
Aziz writes the fantastic Muslim blog City of Brass