Ok, here's my take on things. My personal interpretation of the movie is that the camera is telling the truth - the people may be lying, the memories may be false, but whatever the camera is showing you directly, as opposed to through a story or memory, is the complete truth.
What's this mean? It means that Teddy, most likely, is actually telling the truth in the end. It means Natalie did in fact lie to and use Leonard to protect herself, but then really did seem to feel sorry for him later and help him. It means that the only person Leonard trusted - himself - was the person that ultimately betrayed him. He lied to himself, convincing himself that Teddy was the John G. in question, and killing him for no reason other than a fit of momentary anger.
For all of the appeal of the character of Leonard, I think he really was a bit of a psycho, locked forever in a perpetual hunt for his wife's killer, and killing anyone he thought might have been that murderer. I think he's killed at least three people - the original John G., Jimmy, and finally Teddy - and possibly other "John G."'s as well.
I think the moral of the film, if you can call it that, is that no one can be trusted. You can't trust your friends, you can't trust your enemies, you can't trust your memories, you can't trust "the facts"... and most dishearteningly, you can't trust yourself. Just like Teddy said, we lie to ourselves every day to make ourselves happy, and Leonard was just as guilty of that as anyone else (actually, probably more guilty of it).
I do not think Leonard killed his wife, which apparently several people do. I think he did kill one of the two killers, after stumbling on them raping his wife, and I do think the second one smashed his head into the mirror, giving him brain damage, and I do think he killed Teddy for no good reason.
Comments? Different interpretations? Anything?
(This post was inspired by a conversation with ebess, a post by weetanya, the movie itself which I just recently saw for the first time, and the letter F.)