It's pretty obvious. The person who took the real locket from the cave has the intials R.A.B. I'm putting my money on it being Regulus Black. He ran from Voldemort, he knew he was going to die, and he referred to him as the "Dark Lord" in his letter. I was interested in the whole Mundungus stealing from No 12 Grimmauld Place storyline, so I went back to Book 5 to see if they had managed to find a locket when they cleaned out the room with the Black family tree, and sure enough-there was a "heavy locket that none of them could open". I'm betting Mungungus has currently got it in his big bag o' stolen goods.
Also, I know I'm just in denial, but why oh WHY did she have to make Dumbledore's funeral seem so much like the renewing fire of a phoenix? I just keep hoping that somehow he won't be dead and he'll rise from the ashes to guide Harry again. But no, when you're dead, you're dead *sigh*.Is it just me or does it feel as though there was a lot more that he needed to have explored with Harry? This is all too much.