The recent apps list in Ice Cream Sandwich added the ability to swipe apps out of the list, thereby dismissing them permanently (and as far as I know this is a vanilla function, not a CM/custom ROM one). The documentation and platform highlights don't appear to cover the under-the-hood workings of this functionality, but I'm curious to know what the system is actually doing.
Further adding to my curiosity, I decided to do a quick test: I started up Music on a CM9 install, then backed out of it. I then checked the recent apps list and saw it was indeed there (and in the proper state, based on the thumbnail). I then went into
Settings->Applications and force stopped the Music app, but it was still listed in the recent list, leading me to believe it's not connected to processes lingering in the background.
Realizing now that Music may have been a poor choice, I also tested with the USA Today app. This exhibited basically the same behavior, and it seemed like it was forced to "relaunch" after the force stop (which makes sense) though the thumbnail in the recent apps list didn't reflect this (cached, I'm guessing?).
So, what actually happens at the OS level when you swipe an app out of the recent list? Does it simply clear the app's data out of RAM and garbage collect it, destroying its saved state?