AdamR: It says "all keys are for your own personal use"
What if I
personally give a key to a friend because I own the game already? Am I going to get sued?
Sued? Are you serious? Anyway, I believe they put that there because a lot of people sort of split the bundles in an unethical way. For example, some people download the DRM free installers and trade/sell the Steam keys, or they keep the Steam keys and trade away the Desura keys. As long as you only give away a game that you already own that's perfectly fine.
Foxhack: Though, you only get -one- Steam key for all games.
You get a separate key for World of Goo if you beat the average.