Came across this link via King Lud IC, a gamer / game designer from Virginia:
The fourth wall is the invisible divide that separates the viewer from that being viewed. Subjects that break the fourth wall show self-awareness, such as in Contact DS (not mentioned in the Wikipedia article) when the professor talks directly to you, the player. Depending on how it’s handled, it’s either a cheap gag or a very effective post-modernist trick to change the perception of the viewer.
In games? Mostly a cheap gag. Still, it’s interesting to see the discussion.