As a Dawkins-esque atheist myself, I celebrate Christmas to the T. There is nothing in the current Christmas tradition, including presents, Santa, Christmas Trees, ornaments, Egg Nog, et. al that requires or even hints at Christ himself.
The value of presents, and time with family, and cozing up to the fire and all is far too positive a force to disregard simply because some religious idiots think the date coincides in any way, shape or form with the birth of Christ of Nazareth. Christmas was a co-opted pagan holiday, and still is a far more secular than religious holiday.
EDIT : to add this link; I'm not sure if you're familiar with the "Friendly Atheist" blog, but it's one of my favorites and it was the one I was thinking of when I saw this post. It does a good job of addressing why Atheists and Humanists and Freethinkers, etc., can celebrate Christmas..
http://friendlyatheist.com/2009/12/2...-the-holidays/
On an unrelated note, this is amusing for those of you with religious family:
http://friendlyatheist.com/2009/12/2...-the-holidays/
EDIT2: Links were broken