Patrick Melton and Marcus Dunstan, the writers of both "Saw IV" and "Saw V", have a cameo in the flashback of Hoffman being kidnapped and meeting Jigsaw for the first time. When Hoffman is about to get on the elevator right before getting kidnapped, Dunstan is the second man to exit the elevator (he is in black and has a beard), right after the snobbish woman. Melton is the tall man who exits the elevator last before Hoffman enters.
(Spoiler) Strahm is killed by the walls of the final room crushing him. However, in the original script, rather than the walls crushing, the room was supposed to fill with water, thus mirroring that trap that nearly killed Strahm in the beginning of the movie. Due to practicality reasons, this idea was dropped, though it does explain the air-tubes coming of the glass box. (Which would have been for Strahm to breath.)
At one point, Strahm's head-trap was going to be featured at the end of "Saw IV", but it was eventually decided by the filmmakers that the trap was strong enough that it could be one of the featured traps in "Saw V."
The elderly couple who notices Jill Tuck in the waiting room near the beginning are actually the parents of the film's casting director, and the receptionist in the scene is played by a receptionist who works for Twisted Pictures (which produces the "Saw" films) in real life.
In the beginning of the movie, Seth's trap consists of a bladed pendulum swinging towards him. We are told Seth is also a criminal. Agent Strahm's trap consists of a room closing in towards him. These are elements of the story "The Pit & the Pendulum," written by Edgar Allan Poe, who is widely considered to be the father of the horror genre and detective genre, both of which are used heavily in the film. Another of Poe's stories, "The Murders in the Rue Morgue," is also similar to the film. Ashley is decapitated, while a character in the story suffers near-decapitation. The characters in this story are also in a room locked from the inside.