Can you smell burning oil? If not... it could be just the drain plug is a) loose, b) missing the gasket, or c) has a cracked gasket. I had problem C with my Camry last spring, was a dumbass and overtightened the bolt and had some seepage. So we tightened it even more... problem solved
I just changed my oil today in fact, and it's burning like a mo'fo'- my gen2 people who have taken off the filter before all the oil drained out of it know what I'm talking about! (And for those of you don't... it leaks all over the focken' block and onto the exhaust, also into the exhaust heat shield trapping it b/w the pipe and the shield! Smell lasts some time...)