The guys at IGN have a decent article up about each of the classes we'll see in Valve's Team Fortress 2. The go through each class, list Valve's official description, then give their thoughts & opinions on how each will play out in the game. I'm definitely looking forward to wasting many hours on this one.

"Though you don't earn the powers or weapons of a class you're pretending to play, an opponents' medi-gun will still heal the Spy. You can even be turned invulnerable by the Medic."