I am Joe's...

Subsequently, the narrator then uses this idiomatic device to express his feelings at various times throughout the remainder of the novel. For example:

I am totally Joe's Gallbladder.
I am Joe's Raging Bile Duct.
I am Joe's Grinding Teeth.
I am Joe's Inflamed Flaring Nostrils.
I am Joe's White Knuckles.
I am Joe's Enraged, Inflamed Sense of Rejection.
I am Joe's Clenching Bowels.
I am Joe's Boiling Point.
I am Joe's Blood Boiling Rage.
I am Joe's Smirking Revenge.
I am Joe's Broken Heart.
I am Joe's Cold Sweat.
I am the Pit of Joe's Stomach.

The Jack (and Jill)/Reader's Digest theme was another excellent metaphor of the movie in my opinion. It adds another teaser to the eventual audience realisation that Tyler Durton is infact the Narrator's evil alter-ego.

"I am Jack's raging bile duct"
"I am Jack's total lack of suprise"

Lines like these give us a sniff that the Narrator is depersonalising aspects of himself, the eventual downangle on this is the creation of his nemesis, the big bad boy, Tyler Durton.

Maybe I was tripping at the time, but this stuff kinda began to give away the whole split personality thing when I saw this movie for the first time.

In the oldest magazines, there's a series of articles where organs in the human body talk about themselves in the first person: I am Jane's Uterus.

I am Joe's Prostate.

No kidding, and Tyler comes to the kitchen table with his hickies and no shirt and says, blah, blah, blah, blah, he met Marla Singer last night and they had sex.

Hearing this, I am totally Joe's Gallbladder.

In the book it was Joe, for the film this was changed to Jack. Likewise, Jane was changed to Jill.

Because of Edward Norton's character in the film frequently using these phrases to express his feelings ("I am Jack's Complete Lack of Surprise"), some people (including the crew on the film set) call this character Jack.

I really can't see why, other than for the convenience of having something to call him. "The Narrator " is what I tend to call him, because that's what he's listed as in the movie credits, and in the book he takes the narrative first person perspective. This, sadly, seems to have become quite popular, to the extent that on the DVD he is (apparently) called Jack. I have also heard that in certain scenes of the DVD he is captioned as "Rupert" (this being just one of the many fake names he gives at the self-help meetings.

In actual fact, we never learn the narrator's name. In the film, one scene ends with the Narrator giving Marla his phone number, and her saying that he forgot to put his name. A bus goes past. The scene ends.

In short, I am Joe's ... (or Jack's ...) is NOT a hidden clue as to the Narrator's name. Any search for this name is futile (there is a rather irritating discussion on IMDB which illustrates this quite nicely. "It's Jack", "It's Joe", "isn't it Rupert?"). It's a very funny narrative device.

It was changed from "Joe" to "Jack" in the movie because the Reader's Digest series was, in fact, real, and the producers of the movie couldn't get the rights to use the original names. I think Jack sounds much better anyway.

The original movie script refers to the character as Jack, which is why he is so referenced on the commentary tracks. Presumably, this was done to avoid potential confusion, and also because "Jack" is shorter and easier to say than "The Narrator". Calling him Rupert or Cornelious or Travis is just silly, since in the movie he says outright that he never gives his real name at support groups.

Thinking about it some more, there seems to be a reason for the character's namelessness, beyond the desire to let the audience relate to him more. Until he becomes Tyler Durden, he really is nobody. Certainly nobody whose name you'd remember. In a lot of ways, that's the point of the movie.