I don't think that you'd need 120FPS, I would say halving 60 back to 30 would be just fine unless theres some reason for the glasses?
I suspect that the Xbox 360 could implement a more basic form of 3d rendering by implementing an update in their Ed-ram (forward compatibility) which would be similar to the Nvidia technique and allow them to rasterise the same scene twice with a slightly different camera angle (perhaps as im not 100% sure if im right here) to allow 3D on legacy titles and advanced 3D on future titles which support it.
Tease.







