Couldn't be anybody else. All the facts fit, including Sirius' comment that "he got in so far, and found out that you couldn't back out."
What it tells me is, Harry's going to have to go back to No. 12 Grimmauld Place in Book 7.
I agree that it is Regulus. To me, the big question is who helped him? Does not look as if he could have done it himself, does it?
Yup. That is, presuming good ol' Mundungus Fletcher didn't steal and sell the Horcrux.
I have been wondering if Sirius won't make an appearance of some sort in the future... his story just seems so horribly unfinished.