We just returned (April 3 to April 8, 2003) from Disney World and can report that there were VERY few non-US visitors. We saw/heard a couple of families from the UK and from Australia, but basically no others from outside US. Of course, SARS is here in the US as well....
Additionally, we brought along a supply of Wet Ones Antibacterial Wipes and used them (under any conditions, it's not always easy to get a 5-year-old to wash hands before eating, but this made it simple and fun). Not only did we wash our hands with them, but to be honest we wiped down the arm rests/shade pulls and other surfaces around us on the airplane. We were fine.