Think about it from JKR's point of view. Why have someone steal the horcrux and leave mysterious initials that leaves everyone speculating for a couple of years until HP7 comes out, to reveal that it was someone who's been dead for years and adds no type of new character to the story. Bit of an anti-climax.

Good point though, I'd forgotten the letter said they were long dead...me and my memory eh? Tch!

I'm gonna stick with my thoughts about it not being Regulus.

Now you cna see why I'm not a betting man though......we'll all finish HP7, look back on this thread, see I was horribly, horribly wrong and work out how much I owe people in bets that it wasn't Regulus.