The most likely cause (if it's soot) is from the furnace, either genreated from the fuel or brought in from the outside air. I recommend that you have an analysis done. Go to iaqa.com and find a CIE or AIHA and find a CIH to do an indoor air analysis. It may be minor, but you have a problem that needs professional attention for the sake of the kids, if for no other reason.
I would definitely get the situation checked out by a pro.
[This message has been edited by rpxlpx (edited March 03, 2003).]