For me it ocurred in several stages:
Last year, one of the Warez groups had an installer that featured a still picture of "the gang" accompanied by MIDI music and voice clips of Cartman saying "son of a bitch!" and "I would never let a woman kick my ass!".
Soon after, I began seeing bumper stickers, pen sets, and other merchandise.
Since I do not have cable, I began searching for episodes on the Internet. I soon found www.billzouse.com's "South Mall", and began downloading Real Video episodes. The scene responsible for getting me hooked was "dressing like Hitler in school isn't cool" in the "Pinkeye" episode.
Other favorites are:
Cartman's "ants in the pants" birthday party tantrum
Mrs. Cartman's "I know a kitty who is sleeping with momma tonight!" after Cartman calls the cat a dildo
Chef's "Simultaneous Love" song
"What am I doing here? I'm doing Celine Dion!"
Cartman's rantings when the first graders turn the pool yellow
Cartmans tea party
Cartman's 'bitch song" (of course!)
Mr. Hanky "decorates" Kyle's bathroom
The numerous references to movies such as Titanic, Braveheart, Star Wars, ET, etc.