So, after taking my car in for a leaking main seal
http://www.e90post.com/forums/showthread.php?t=316748, the mechanics noticed that one of my head bolts was broken as well, and seeping oil out of the hole.
My car is back at the shop tonight while they try to extract the remaining part of the bolt.
After some research I've found that it seems to be happening to quite a few N52 engines:
http://www.bimmerfest.com/forums/sho...d.php?t=280142