Part 2 of the mugglenet.com interview with JKR is up and she gives some really great answers to things. She kind of confirms that Harry will not be coming back to Hogwarts next year as a student. She said the match with Luna commentating was the last Quidditch match she will write.
Anyway, this is what she says about Regulus:
MA: R.A.B.
JKR: Ohhh, good.
[All laugh.]
JKR: No, I'm glad! Yes?
MA: Can we figure out who he is, from what we know so far?
[Note: JKR has adopted slightly evil look here]
JKR: Do you have a theory?
MA: We've come up with Regulus Black.
JKR: Have you now?
MA: Uh-oh.
[Laughter.]
JKR: Well, I think that would be, um, a fine guess.
MA: And perhaps, being Sirius’s brother, he had another mirror –
JKR: [drums fingers on soda can]
MA: Does he have the other mirror, or Sirius’s mirror —
JKR: I have no comment at all on that mirror. That mirror is not on the table. [Laughter from all; Jo's is maniacal.]
MA: Let the record note that she has drummed her fingers on her Coke can in a very Mr. Burns-like way.
[Laughter.]
JKR: Oh, I love Mr. Burns.