Popularized by the SomethingAwful and World of Warcraft forums in March of 2007, it's starting to disperse onto more websites as of May.
Rick Rolling can also be a real life act of the music clip - normally by playing the song and having an impostor sing or dance to the video. Thus "Rick Rolling" the unsuspecting audience.
It is popular on video's that promise exclusive footage - such as a GTA4 gameplay, Or Halo3 exclusive.
Clicking the link to a video of Rick Astley's "Never Gonna Give You Up", thus being Rick Rolled.