The word Xmas is fine written, as long as it's not pronounced Exmas when spoken. X has been used as the symbol for Christ for almost 1000 years, by all religions, and most of us have seen the word Xian used in RE at school. The X in Xian is the same 'abbreviation' as the X in Xmas.
Actually, as an atheist, I couldn't give a hoot how people pronounce it, but it seems to wind some people up.