Very interesting question!
So I googled...
http://www.triathletemag.com/story.c...92&pageID=1705
I wondered about quanities of sodium and other electrolytes. It appears that sodium is lost through sweat a great deal more than the other 'lytes.
EDIT: a key point is that excessive sweating and/or salt residue isn't necessarily a bad thing -- see your doctor if you're concerned -- but being aware and replenishing is good.