Originally Posted by roadkill

Where did this tradition come from? Is it pre-Christian?

I don't know specifically, but I'd hazard that the tradition of feet = dirty is widespread and ancient in the middle east. Hence the throwing of shoes at Bush, and now Blair. Plenty of historical precedents for all that, and I'd guess it goes back to the Mesopotamian city states. I don't know if the washing part has any precedent; as a conscious demonstration of humility, though, it was adopted by Christianity on Biblical precedent.