Not an original theory, I know. But he was mentioned three times "incidentally" in HBP--Jo doesn't typically do things like that by accident. Also she's said no new characters will be introduced, so that means it's someone we already know about. There are others, but no one that's been fleshed out as well.
Some have also pointed out that Regulus would have had the means to get the locket as well--i.e. Krecher who could have 1) gotten on the boat, 2) drank, made Regulus drink, or used a special brand of house-elf magic to get rid of the potion, and 3) kept the secret.
And then there's the locket in Grimmauld Place--the one that they find early on in OOP when they're cleaning. Merope Gaunt's perhaps? Me thinks so. After all, Jo can't make all the horcruxes as hard to find as the one's Dumbledore found. She has to make a few at least a little easy, otherwise Harry wouldn't survive.
I think the real surprise here won't be who R.A.B. is, but why and how he stold the horcrux....