My fiancee recently got herself a Wii Fit. I awoke (rather tardy) this morning to find her doing step aerobics.
Sharon: "I usually just switch it over to regular TV while I'm doing this."
Me: "Couldn't you have just done that without the Wii?"
Sharon: "Yeah, but I'd usually just be sitting there like you (sprawled lazily on the couch)."
The Wii remote has a built-in speaker, so the game can coach you along even if you've switched the TV to a different channel. The combination of hearing the remote's encouragement, knowing that the game is tracking your progress, and knowing that you would feel guilty if you spent money on the Wii Fit and never used it is enough to compel people to exercise instead of just sitting around.
Perhaps I'll write a longer series about this concept. If you have any ideas, feel free to e-mail me or leave a comment.