First things first, don't panic. It is a recognised issue with the engine, but BMW claim it doesn't damage the engine; so it's more annoying than anything. If you use the car regularly, you should rarely experience the problem.
Secondly, I assume because you're not sure what warranty you have, that you didn't buy from BMW. That does make things a bit more difficult to be honest.
What you can do though is make sure you take it to a garage that has experienced the issue previously. They are then more likely to believe and/or understand the issue you're experiencing.
As PVR says, definitely record it on a video camera if you can - this just helps with presenting the problem to the dealer. I'm not sure where you're based, but Dick Lovett Swindon took my car straight in, diagnosed it, and fixed it; all with no hassle. They will have to replace the HVAs (Hydraulic Valve Adjusters) first, as per BMW instruction. You then, like me and Steve, require a full cylinder head replacement.
Be reassured that ALL 2.5si and 3.0si engines suffer from this issue, and I don't believe anyone has experienced a dead engine due to oil starvation in the lifters.
I hope that helps somewhat, but Steve (srhutch) is the expert when it comes to this.