Tyler did it... with a broken collar bone.
Ullrich tried a couple of years ago, twice. He couldn't do it. CSC (Jens Voigt I think) helped bring him back
Technically, Rasmussen did it yesterday, but he wasn't really a GC contender. Nor was Tyler when he did it.
I think it happens so rarely because so rarely is anyone strong enough. The other teams didn't let him go today, they couldn't bring him back. This was obvious on the final climb, when even though he was solo all day, no one could make progress on him, except Sastre, who got about a minute, than lost most of that on the finishing descent.