If you were living in New Orleans now I'd say you just had condensation from cooler temps and high humidity, but in Oakland I would not expect that to be the case. I think I'd remove the valve cover, remove the gasket, clean both the head perimeter and the valve cover inside and out well, replace the gasket and see what happens. After you do this you may (emphasize the word "may") be able to get a better idea where this is coming from. What is the fitting on the side of the valve cover?
One other thing you can do if you have a skil saw (saber saw, call it what you wish), some 3/16" thick aluminum sheet and a metal blade is make some spreaders (like these):
http://i1154.photobucket.com/albums/p535/JonGoodman/valve%20cover%20spreader%201.jpghttp://i1154.photobucket.com/albums/p535/JonGoodman/valve%20cover%20spreader%202.jpgThey're about 1/4 inch wide (except for the screw area) and about 13 inches long or so. They bend in the center to spread the downward force and with them you will never have a leak at the valve cover again.
