I have been pondering this very question for a couple of months now after I noticed the rise in villains becoming heroes. Happily Ever Afters are not just the realm of the honourable anymore; particularly with in paranormal romances I have noticed an increase in the bad guys getting the girl. In fact this has become one of my favorite tropes (read Lothaire by Kresley Cole and you'll probably be a convert as well). To me, the appeal of this trope is that the villains-hero retains an element of his evilness, yet our heroine can often see past this evil persona to the person beneath. Most villains are, after all, not born evil, but instead become evil due to circumstance.
My thinking is this: if villains are becoming heroes, is there hope for a villainess becoming a heroine? If so, how are they being portrayed? Are authors making the former villainess completely redeemable? Do they, after years of so-called “evilness,” have a change of heart and become “good.” Also I'm using quotation marks because defining someone as purely “good” or purely “evil” seems a little to simplistic; instead, what this trope allows the writer and the reader to explore is more complex, three-dimensional characters. Characters who have, sometimes, taken the wrong path, or multiple wrong paths, or even characters who have no interest in being “good” in the traditional sense of the word.