The way 3D works is that there's two images, one for each eye (just like in real life). The differences is how they make sure that the image that is meant for each eye is only visible by that eye.
Movie theaters use a polarizing technique where they light from the projectors have a polarizing filter on them and then the lenses of the glasses are a polarizing filter that separates them out. Those glasses are cheap.
Then there's what most 3D TV's have, which is active shutter technology--the TV switches between the two images very quickly, and the glasses are electronically synced with the TV, the glasses block one eye and switch so that when the left image is on screen, the right eye is blocked and when the right image is on screen the left eye is blocked. The reason for the 3D TV is that they have to have a high refresh rate, if the TV isn't fast enough then you end up seeing stuttering, or ghosting, also the TV has to have the hardware that wirelessly links to the shutter glasses.
Anyways, that's a short explanation of everything.
For this, I'm wondering if they'll go back to the computer files for the visual effects to recreate the 3D image. With a 3D conversion for most shots you can simply cut out elements in the shot and layer them based on where their distances should be, but in other cases a single object has parts that are close and far away from the camera (like something poking out at you) and that requires 3D recreations to create a proper 3D effect. So for the stuff that's computer generated they already have the data to make it 3D, hopefully they do that. I think that's what they did with the 3D versions of Toy Story 1 & 2