Maybe so, Jeff and WYSIWYG, but it's exactly what I write when I want to de-religiate the word. For you, the X "represents a cross", and "represents a letter in the Greek alphabet which is the first letter in Christ", but y'all are stretching things a bit to keep it how you want it to be. I'll bet neither of you actually use "xmas" to represent the holiday. Because actually, it doesn't.

"Xmas" is a colloquially comman way of writing the name of the holiday and removing the myth-focus. Its very ability to remove Christ from Christmas is what makes it a potent word.