Well, it depends on whether you want to go with "official" interpretations of things, or with the interpretations that you yourself come up with. iCEBaLM has some good points, and so do you, but I think that the answer lies somewhere between -- and that neither one of you really has the full answer. I don't think I have the full answer, either, but I've got some textual evidence, at least.
I haven't seen any of the DVD commentary tracks, but it strikes me that discounting an interpretation that is arrived at through logical interpretation is a form of closing off discussion -- and it also strikes me that quoting someone else's interpretation isn't precisely a form of textual proof.
Personally, I think that the scene at the end of the movie reinforced Project Mayhem (and Pitt's Tyler) and discredited it at the same time. When Norton's Tyler shoots himself to shut up the part of him that the audience knows as "Tyler", it's an act of destruction -- a conscious, deliberate act of destruction. (Attempted suicide is usually classified as destructive, wouldn't you say? ) Considering that from the middle of the movie onward, Norton's Tyler was fighting against what he felt was destruction that had gone too far, trying to stop it, this was significant as hell to me -- the moment that gun fired was the moment that Pitt's Tyler won, even though he had been banished.
The irony that I found in the ending was that in order to banish the more primal, destructive side of himself, the more 'normal' part of Tyler (Norton's Tyler -- damn, this is hard to talk about) first had to embrace that destructive side of himself. The act of shooting himself in the head to banish Pitt's Tyler is the ultimate in giving in, of directing violence inwards to keep from directing violence outward. It was something that was being set up all through the movie, from the moment that Bob was killed onward -- the dichotomy that sometimes, it's better to internalize your violent nature than to let it out and let it loose. On the surface, the movie was advocating the exact opposite -- glorifying Project Mayhem, glorifying the very concept of "Fight Club" itself. However, the device of the flashback, and the fact that the entire movie is told via Norton's point of view while he's sitting in a chair with a gun in his own mouth, sets us up to accept the opposite.
After Norton's Tyler shoots himself in the head, and Pitt's Tyler disappears, the rules change. Norton's Tyler, in that one moment, has accepted the same kind of destruction that Pitt's Tyler had been advocating the entire movie -- only turned inward, to avoid turning it outward. That, I think, is why Norton's Tyler turns to Marla and says "I'm okay". He's found a way to integrate the two halves of his personality, and he's found a way to satisfy both the destructive urge from Pitt's Tyler, and the sense of social responsibility from Norton's Tyler. And yes, if you really push it, that moment can be read as an acceptance of personal responsibility and maturity, rather than pushing one's personal problems onto society and externalizing them.
Radical change can only come from radical people, as iCEBaLM said. The movie's pretty clear on that -- and, if you take out the framing device of the building and the explosions, it's pretty clear that's textually supported. Adding in the framing device and considering the movie as a whole, with the ending, you get the additional commentary that sometimes radical change isn't possible without radical personal change as well. I think that the final, overwhelming message of Fight Club was not that violence can work and that it's time for the society we have to crumble, but that a house divided against itself cannot stand. The very fact of Tyler-prime's insanity, the fact that one character is played by two people and one of those people wants to destroy society while the other one wants to participate in it, indicates that duality like that can never be successful.
I could keep going, and add in some of the significant events of the center of the movie, but this is long and rambling enough, and I was just talking about the end. ^_^
"I'm the screen, the blinding light; I'm the screen, I work at night. I see today with a newsprint fray, my night is colored headache grey, don't wake me with so much..." -- REM
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