Here are some things I noticed in the movie after watching it a number of times. Feel free to add to this discussion things or scenes that may have hidden meanings, etc.
1. One of Tyler's jobs was working in a movie theater and splicing movies. There are a few scenes where splicing is apparent. Early in the movie when 'Jack' is talking at the copy machine in his office, an image of Tyler is spliced for a second in front of the machine. When Jack is in his hotel room on one of his business trips, there is a commercial for the hotel on the TV with hotel staff saying "Welcome". Tyler is one of the hotel staff in the commercial. The most famous splice is at the end of the movie while 'Jack' and Marla are watching the destruction of the building, and a, ah, questionable body part is spliced into the scene.
There are other things I noticed, but I'll hold off for now and see if anyone else has some to share. Maybe it will prompt some more discussion on the movie as well.
Some other hidden things in the movie that I discovered from watching it several times:
1. When Marla and the Narrator are speaking in the street and he puts her on the bus, there is a movie theater sign for "Seven Years in Tibet". Ironically enough, Brad Pitt starred in this movie.
2. When the Narrator is at the doctor's office, and he tells the Narrator about visiting some support groups, and says "That's pain", Tyler appears next to the doctor.
Fight Club has a multitude of nuances that one can not appreciate until one has watched the movie several times. There are a total of three apperances of Tyler being spliced in prior to seeing him on the elevator. First is the one by the copier, the second is the one by the doctor, and the third is the one by Marla as she walks up the street after one of the group meetings. My favorite part of the whole movie, however, is the Warning in the begining...right after the obligatory FBI warning there is a Tyler warning, if you haven't seen it, check it out.