The fine folks at Digital Foundry have a great write-up on the technological trickery going on under the hood of the PlayStation 3 to make existing games 3D.
If the following excerpt interests you, I suggest you read the full article:
Step two is to apply the convergence to define the maximum depth of the image, the maximum positive parallax," Bickerstaff says. "It's a 2D X-axis translation in screen-space and we move the left image to the left and the right image to the right. For our games we've used a 1/30th screen width as the default parallax.
For the rest of us, there is the report that some games will take a framerate and graphical quality hit during the 3D conversion. WipEout HD originally ran at 1080p and 60 frames-per-second. The 3D version, which will be available when 3D support launches, runs at 720p and 30 frames-per-second.
MotorStorm: Pacific Rift, which will get a one-level 3D version, already started at 720p at 30 fps. To hit 3D, the designers used the game's split-screen mode codebase, which reduced the graphical fidelity to allow the game to render two images at once. Then, both cameras were placed on the same image to generate the two required images for 3D.
An earlier article at Digital Foundry confirmed that the 3D version of Super Stardust HD will run at 720p and 120 frames-per-second (60 fps for each eye). To achieve that, the developers behind the game put an additional year of work into improving the game engine. It was not something that came about by flicking a simple switch.
It is becoming clear that taking existing games, engineered for 2D, will lead to quality or framerate hits. If developers dive back into the codebase to optimize for 3D, higher quality is possible, but games designed from the ground up for 3D will look and perform better. The question is how many developers will be willing to put in the extra time developing for 3D when it is a PlayStation 3-exclusive feature that requires a 3D-ready television.