My take.I hope it helps.

Rating: 9/10 (16 January 2011)

Nina is imagining Lily.
Scenes/moments that confirm my suspicion:
1.After seeing her blurred,dark reflection in the subway,Nina sees the back of a person in a black coat.That person is not Lily-its Nina(or her dark side).Pause the movie so you can see Portman's face. 2.Lily enters the dressing room,says something(not in the script) to the girls and there is no reaction(yes,girls turned when she entered but it could be because of the noise in the hole and then Nina's hallucination takes over) 3.Rehearsal scene soon after:Thomas:...,,But which of you can embody both swans? The white and the black?,,Right after that line we see a short shot of a Thomas's reflection,split in 2 mirrors(suggesting split personality or ,,dissociative identity disorder,, of Nina to come?).In Wikipedia article about this disorder,it says that many ppl suffering from it were sexually abused as a child.Hints about sexual abuse of Nina:Nina's mom offering Nina to lick cream from her finger in an unpleasant manner.Nina blocking the door in few occasions in a manner that suggest fear from mom.And finally,scene where Lily is going down on Nina where we can see face of someone else on Lily for a split second...pause that scene and look that face-Actress playing Nina's mom made to look younger?Now read the next part of the script: -Erica tries to enter Nina's room but the door won't open as Nina has placed a piece of wood between her wall and door- Nina Sayers: Don't come in here! Erica Sayers: What's this? Nina Sayers: It's called privacy! I'm not twelve anymore!(was abused when she was 12?) Erica Sayers: You're not my Nina right now! Nina Sayers: Leave me alone! 4.Nina is trying to dance for black swan,Thomas is yelling/putting pressure on her.As she is spinning we are looking at Thomas's face from Nina's eyes.We see it in brief after each of Nina's rotation.NOW,look at the last shot of Thomas's face-Its going down in a strange manner(NINA HAS FAINTED-but we don't see it because hallucination continued where reality stopped)Lily enters and Thomas says:,,Well,good of you to join us,,(he is speaking to Nina who woke.)HA! 5.Lily and Nina alone in the dancing hall.Lily lights a cigarette. Nina Sayers: You can't smoke in here. Lily: Well, I won't tell if you won't.(Well DUUH,you are living in Nina's head ) Near the end of the movie,Nina returns the stuff that she stoled to Beth(Last thing she puts on the table is a pack of cigarettes 6.Nina and Erica are talking and the talk is becoming unpleasant.They hear a doorbell and Erica opens.She returns after few second and says that its nobody.Nina opens again and starts imagining Lily.Notice how mom doesn't talk to Lily and is acting like there is nobody there. (Someone actually rang and exchanged few sentence with Erica then left.It was a mistake so she says,,nobody,,)So Nina goes to town with her imagined friend.I am having hard time explaining to myself the interaction between Nina,Liliy and two guys in town...I have idea but this post is getting to long so I leave it to The Lounge 7.Guys in the club ask the girls:Tom: So you two sisters? -at the same time- Lily: Yes. Nina Sayers: No.(Me:hmmmm) Lily: Blood sisters.(Me:hmmmmmmm) Nina Sayers: We dance in the same company.(Me:ahhhh WTF,for ***** sake,I will figure this out with the help of The Lounge) 8.Scene where Nina and Lily are back in Nina's place after night out.Erica is awake and starts to question Nina about her night out.NOTICE the interesting shot of Nina and Lily SPLITTING then mom asks: Erica Sayers: Where have you been? Nina Sayers: To the moon and back.(we see Lily as she whispers ,,...and back,, in the same time while Nina is answering to mom...conclude for yourself ppl heh.Also,mom seems oblivious to the presence of Lily...strange?-VERY. 9.Scene where Thomas and Nina are talking about Lily...Thomas:I know it's been a struggle. But you just had a break through this morning. Tomorrow's yours, just give a great performance and you won't have to worry about Lily or anybody else. Now, go home and rest. ,,I know it's been a struggle,, maybe means that Thomas is aware of Ninas mental issues.,,break through this morning,, meaning:Erica called to let him know that Nina is sick but she decided to show up and fight with her illness (definition of break through-,,An act of overcoming or penetrating an obstacle or restriction,,).,,Lily or anybody else,,(maybe Lily isn't Nina's first imaginery friend/alter ego. 10.And finally,Nina stabbed Lily(No,Nina stabbed Nina)...I hope this post was helpfull.