As far as I'm concerned, it's Buffy's version of Faith. Faith is probably already trapped in her own dreams.
About the "Not gonna. It's yours" line, Joss said:
"Its not really a transfer of Slayer-Powers, more just, emotionally, I wanted to take Buffy and Faith to a place... where their relationship could have been rather than just play them as arch enemies. There was a great deal of love between them and that would manifest itself when Faith was no longer in a position to be attacking" "
I was thinking about that line some more, and it still didn't gel for me properly. It felt like a bigger deal than most interpretations would suggest. Whether it was interpreted as Faith forgiving Buffy, Buffy's own stressed out head trying to make a plan or Faith's last desperate attempt to protect her friend.. it just didn't work out without a remainder.. Then a few weeks ago, from the most unexpected of sources, I found my favourite interpretation so far. televisionwithoutpity, which is funny but, as the name suggests, utterly merciless in its snarkiness, reckon the dream sequence is one of their favourite scenes ever. Remember, these are people who found stuff to sneer at in The Body, but in a moment of dead seriousness, they said this about the dream:
"It's said that everyone in your dreams is actually you, and in light of the dialogue between Faith and Buffy throughout the third season about how Faith is the embodiment of a certain darkness in Buffy herself, I think that's what the viewer is meant to take from it. It's clear also that Faith's baggage is just that, and that Buffy's going to have to 'carry' it now, because trying to kill Faith is very"¦well, Faith, and all that that implies. It's also said that, in dreams, apartments signal you to avoid rivals, and that where windows symbolize opportunity, a broken window symbolizes an opportunity lost. So, there's all that stuff. But it's also possible that, much like in 'Restless' and in real dreams, we've got a number of red herrings here that don't mean much of anything." -- Sars
So, put simply, they reckon this scene is Buffy's way of acknowledging her responsibilty to Faith. In her lust for revenge rather than justice, Buffy has let the Faith Within loose now and can't go back. So she takes on all the guilt and pain Faith's been dragging around for so long because she's comatose and can't do it herself anymore. I'd like to think so...It would make Buffy far more sympathetic to me. No wonder she treats Post-Coma Faith as she does: For nearly a year she's been dealing with her own guilt burdens along with Faith's... It also leads me to believe that the whole scene is indeed in Buffy's head only, and that might explain why DreamFaith acts and dresses so oddly, even for a dream... But even if it was Faith, and she had forgiven Buffy, bear in mind that between that point and her re-emergence in This Years Girl, she's spent 8 months (God knows how long in her dreams) seeing Buffy kill her and the Mayor endlessly. Her lines during the picnic dream suggest it isn't the first time she's seen that particular dream either...