against wallhacks etc.
just got that idea from watching hough at dreamhack answering a question about cheating (three pillars being: code-consistency checks, VAC, community tools such as kick/ban commands for admins).
well it's actually nothing new, i've seen it happen in other games before. the idea is that a client is forced to randomly make screenshots when playing (you don't have to do this very often, i guess it's more important that the timing is not predictable) which will be stored on the server for future reference.
now if some player is accused of cheating, say wallhacking, an admin can bring up a few screenshots associated with this player to have a look at the game from that players perspective. if there is anything non-legit going on, he can decide to ban that player.
this will of course not cover all possible cheats, but i think it will cover the cheats which are the hardest to trace because you cannot identify them just by observing the player as a spectator.
taking a screenshot might of course cause a small lag, but it might be enough to do like zero to one screenshot every game. if someone is long-term cheating, chances are he will be caught sooner or later.
now i'm no expert at this so i don't know how difficult it would be to trick this system in itself by sending a different screenshot to the server, but maybe there are ways to prevent this from happening. e.g. the server could remember the area the player was in while the screenshot was taken, so the admin can see if this matches with the screenshot.