My girlfriends 2007 X3 3.0si also had the same leak.
She started complaining about constantly smelling burning oil. I would check under the car for leaks but there were none. I thought it was the valve-cover gasket that needed to be replaced so I ordered one online..
Once I began taking everything apart I then found where the leak was coming from!
I called the local BMW dealer which luckily had the oval-shaped gasket in stock, the piece cost approximately $35.00. The dealer also mentioned that this gasket does give out quiet often on these vehicles.
Below is a link to the part on realOEM. The cylinder-piece is an "actuator" and all you need to replace is the gasket itself.
http://www.realoem.com/bmw/showparts...36&hg=11&fg=15
It was incredibly easy to replace! Sure saved a LOT of $$$ by doing it myself. I think the hardest part was cleaning out the oil from all the little reservoirs it had pooled up in.
I've attached two pictures which show the parts removed, where the leak came from and where the oil pooled up in.
If anyone needs more pictures or a fairly decent write-up on how to DIY just let me know!