I loved Jamin Brophy-Warren's piece on the perils of co-op play. He talks about how his brother was always the aggressive, pesky, pain-in-the-neck player out for himself whenever they'd team up to play a game, and how he experienced this same behavior at E3 when trying out the new Super Mario Bros. Wii game's co-op play on the show floor.
As a researcher and designer, I tend to dwell in a utopian vision of providing opportunities for people to co-create really cool and connecting experiences, and often underestimate the little devil in each of us that is in fact cultivated in the gamer ethos of competition and boundary exploration.
The nicest people can take a turn for the worse when they play a game. I summarily refuse to play chess or Risk with my husband for this reason. But a little rough-housing and aggressive 'out-for-me' behavior can add spice to the experience. Our lab's game Wriggle! for example, seems to inspire friendly physical jostling for advantage, even among lab members.
It's an interesting aspect of game design--creating fun co-op play, while anticipating (and maybe trying to ward off the least fun variants of) aggressive and undermining behavior.