Probably the cleverest "solution" I've heard of was one that won by stepping outside the game. There was a prisoners' dilemma competition a few years ago -- I can't find a URL -- in which teams would submit programs to compete in a tournament, with each round consisting of many games against the same opponent. Whoever had the best record at the end won. The obvious strategy in the absence of any other information is to always inform. But if you know a bit more about your opponent...

The winners submitted many solutions that cooperated. Basically, all their programs would start out with a certain sequence of moves identifying themselves either as "winner" or "loser". The "winner" program would, after the signalling period, always inform. The "loser" would, if it detected the "winner" signal, always refuse to confess. Otherwise, it would always inform. Thus their single designated "winner" was able to rack up a huge number of points playing against their other entries, while only losing a small number of points during the initial signalling. Great stuff, IMHO.

/s