Omen started it off -- after stepping into a tanking role, he noticed that a certain Paladin healer had really bonded with him in terms of keeping each other up and running, and it really made them both better players -- the tank was more willing to step up when aggro got lost, and the healer had more reason to keep up buffs and rely on the tank, even at his own expense. Resto, from the other side of the spells, agrees -- even out of raids, the healer there will send the tank potions and go out of their way to keep both together. And from my time raiding as a Resto Shaman, I was always thrilled when I got to be in the same group as the tank I was healing, and got to Earth Shield them and spend my trinkets just to keep them up.
It's not the only major relationship in the game (there's also a nice relationship between the tank and the rest of the melee and DPS, as well as the buffers and the buffed in a raid group), but it is an interesting, minute one, and it's something pretty specific to these MMOs that we play. Playing together isn't just fun and games -- by building bonds with other players in other roles, we both become better at the roles we play.
Update: Just in case, like Ratshag, our little hint on the picture wasn't enough for you, the two characters in the pic above are another fairly well-known tank and healer combo, Tree of Life and Pretty in Plate. You try to hide a subtle little easter egg in there for those of us who read all these WoW blogs, and Ratshag won't let you get away with it. Thanks for keeping us honest.