The more I think about it, though, the more I'm convincing myself that Lily had a hand in the theft of the locket horcrux. I finally bookmarked this thread so, in a few years' time, I can enjoy how wrong I am about everything!
I don't think that theory works at all. First off, apparently the whole of what remains of the Order thinks Regulus died a coward. Second, even if he did heroically decide to try to thwart Voldemort, it's just not likely he'd ask Lily. She's a couple years older than him, was in a different House at Hogwarts, married his (probably hated) older brother's best friend, and was Muggle-born. Remember how the Blacks, other than Sirius, felt about Muggle-borns.
Also Lily would have left some sort of clue, or told someone else, about something so very important, no?