Inspected the car underneath and it's not the spark plugs or the cam cover.
Oils seems to come from a top tube.
The likely candidate is now the oil fill tube. It can be cracked and leaking.
The following photos are not from my car.
http://docwilenracing.wordpress.com/2011/02/15/boxster-oil-leak/
Soon I will get the top into service mode and inspect the tube on my car, but I'm pretty sure that it's nothing serious and that it's cheap to repair.