Got here from four_lobsters. I have a different take on what slash "is," so this might not totally jive with what you said, but...

If we take slash to mean "homosexually-themed writing" (not erotica!), I believe slash has already evolved beyond a strict definition of fanfiction - albeit not necessarily in published literature.

I think it's worth thinking about the point at which a fic is no longer considered a derivative work from existing media. Take for example uluthiel's "Another Hunger." It's purportedly a LOTRIPS fic, but when the setting is dramatically altered and all of "canon" besides the names and physical appearances are changed, can it still be called an AU? Couldn't she conceivably literally do a search and replace of the names and then send it to a publisher (without getting sued)? Yet this original work reads as good slash – none of that crap with turning the characters into girls, but at the same time, it doesn't exactly read like something out of QAF. If you look at it this way, then I think we already have tons of "original slash" floating around on the web.

At the same time, if you're talking about crossing the divide into popular contemporary literature, then no, I don't think that's happened yet. But with the sheer numbers of slashers, it's probably only a matter of time ^_^