Not to be a pain, but Biggio successfully transitioned from C to 2B to CF. I seem to recall talks about Pudge moving to 2B as well a few years back. Basically,I think catchers today are a lot more athletic than in years past, and more than a few of them are able to transition to other positions.
While I think it should be extremely hard to do, I don't think it should be impossible in the game.