...my valve cover leaks all the time... but that could be because i damaged it a bit when i put it on, and i've only recently put some sealant around where the seal goes over the cams. It doesn't leak into the timing belt area any more, but i think i've still got some problems around the exhaust manifold side of the cover.
When you put the cover back on, maybe the gasket slipped out and got smashed a little. Maybe after driving around a little it expanded/settled differently and that's what's giving you problems? After you initially put the new gasket on, you're supposed to make sure the cover is still on there tight after you've driven around for a little. I'm sure you knew that and probably already checked it, but i didn't!

I should go do that...