Imagine if, in one timeline, you could have
So if you're going to a ballgame one day, maybe you'd follow everybody in your regular timeline + your Los Angelino friends and a couple of searches around the California Angels (I still call them that). You'd follow your mentions, but an hour after the game you'd want to turn that off for a couple of hours. Then when you got back to your hometown, you'd switch gears and have your Windows Mobile friends (a list) plus your regular timeline plus mentions. All programmable of course. Weekend mode is just a subset of your timeline (a list) + mentions, no search terms that happens to start on Friday afternoon and switches back to whatever you have scheduled for Monday morning.
I also like the "weekend mode" idea. http://inessential.com/2009/12/13/weekend_mode_for_twitter_client which is really "just" an extension built on the programmable Twitter client aspect.