And now to address the big, shiny, somewhat phallic V-2 rocket in the room.
Wallace wrote a big, funny post-modern book that skewers American civilization. So did Thomas Pynchon (c.f. Gravity's Rainbow). Aside from frequent parabola references, that's pretty much where the similarity ends.
Gravity's Rainbow is basically a pilgrimage. Pynchon's characters move from Point A to Point B.
Infinite Jest is static. Wallace's characters are locked in their own skulls, islands unto themselves.
Gravity's Rainbow had some long strange trips, baby. When faced with evil Nazi rocket scientists and the death cult techno-conspiracy of Western Civilization, why not get high?
Gravity's Rainbow thundered with the voice of the paranoid Author God. Infinite Jest doesn't.