Who put the X in X-mas?

Simple answer: X-ians did.

From Urban Legends Reference Pages at Snopes.com:

Claim: 'Xmas' is a modern, disrespectful abbreviation of the word 'Christmas.'

Status: False.

Origins: The abbreviation of 'Xmas' for 'Christmas' is neither modern nor disrespectful. The notion that it is a new and vulgar representation of the word 'Christmas' seems to stem from the erroneous belief that the letter 'X' is used to stand for the word 'Christ' because of its resemblance to a cross, or that the abbreviation was deliberately concocted "to take the 'Christ' out of Christmas." Actually, this usage is nearly as old as Christianity itself, and its origins lie in the fact that the first letter in the Greek word for 'Christ' is 'chi,' and the Greek letter 'chi' is represented by a symbol similar to the letter 'X' in the modern Roman alphabet. Hence 'Xmas' is indeed perfectly legitimate abbreviation for the word 'Christmas' (just as 'Xian' is also sometimes used as an abbreviation of the word 'Christian').

None of this means that Christians (and others) aren't justified in feeling slighted when people write 'Xmas' rather than 'Christmas,' but the point is that the abbreviation was not created specifically for the purpose of demeaning Christ, Christians, Christianity, or Christmas — it's a very old artifact of a very different language.

I've seen a lot of "War on Christmas" outrage about this from various people lately. But it's simply not a secularization of Christmas. It's not common knowledge what the X in X-mas stands for though, so it's forgivable. Less so when journalists propagate the mistaken take on it. From a CNN commentary today:

Because of all the politically correct idiots, we are being encouraged to stop saying "Merry Christmas" for the more palatable "Happy Holidays." What the heck are "Seasons Greetings"? Can someone tell me what season we are greeting folks about? A Christmas tree? Oh, no! It's now a holiday tree. Any Christmas song that even remotely mentions Christ or has a religious undertone is being axed for being overtly religious. And I'm sorry, forget X-M-A-S. Malcolm X? Yes. X replacing Christ? No.

Personally I've got no problem with businesses trying to appeal to all customers, not just Christians. It makes good business sense. From the outrage you'd figure that businesses were excluding Christians or Christianity when they say "happy holidays" when they're simply including them and everyone else who might be celebrating. I don't know about this guy, but I could honestly give a crap if a clerk agrees with any religious or philosophical or political viewpoints of mine. I sure as hell don't need him to affirm my beliefs.

If Sears doesn't have a sign recognizing my various beliefs are my beliefs threatened in any way shape or form? No. I hardly need a marketing campaign or some clerk's scripted lines to feel okay about my personal belief system on any topic.

This seems to be yet another "good ol' days" issue where people enjoyed the previous eras where they were exclusively pandered to at the exclusion of others. Christmas will continue as long as people continue to celebrate it on their own. If the tradition cannot survive without marketing campaigns or government schools pushing it... then the so-called "War on Christmas" is being waged and lost before people even step out their front door.