I'm sure that 2D versions will always be around, and that many movies will probably never get 'dimensionalized'.
I'm with you, I wouldn't particularly want to see 'Casablanca' in 3D, but something like 'Forbidden Planet' could be fun from the back catalog.
Anyways, it seems that Ridley is actually planning two 3D 'alien' prequels now! - http://www.collider.com/2010/04/23/ridley-scott-confirms-alien-prequel-will-be-shot-in-3d/