Christmas as we know it is really noting more than an amalgamation of many secular and pagan holidays. It was standard procedure around the turn of the last millenium, for the emerging catholic church to kind of shuffle their observances to coincide with the local pagans. It's a nice bloodless way to increase the rate of conversion.
The holiday that most americans and europeans now celebrate as christmas has very little to do with the actual birth of christ, and more to do with the observance of the themes that the birth of christ may have introduced.
While you may feel justified in a militant response to a "merry christmas" if you aren't christian, you are in fact, only amplifing the sort of unjustified unpleasentness that you are accusing others of. It would be just as easy, as RimRod pointed out, to reply with a quiet "Happy Channukah."
Christmas really isn't about christ anymore. Ask any group of toddlers who christmas is about, Santa or Jesus? I would bet money that most would answer Santa and would be unaware of any relationship at all to christ. Christmas is about presents now, christmas is a season, not a holiday. A "merry christmas" is tantamount to wishing a happy new year, which by the way is a holiday in many places and doesn't always fall in the Julian January.
The christians are the ones who should be offended, it was their holiday that was perverted.