Whatever Sony's promises may have been regarding stereoscopic 3D performance on the PS3, we're going to get what we get. And so far, that's 1080p content at 60fps (2D) turning into 720p content at 30fps (3D).
(Yes, I'm confusing things a bit by calling Hz measurements fps when technically there are two 3D frames for every one 2D frame. But the point is, gamers will see 30fps worth of smoothness, not 60.)
The game serving as our case study is Wipeout HD - one of the few PS3 titles that was rendered in native 1080p/60fps in the first place. Even this title, backed heavily by Sony itself, has to scale things back big time to run in 3D.
That's understandable - you are asking a game, after it's been developed, to render two frames (for the stereoscopic effect) where one would do before. PLUS, you're most likely asking the PS3 to do a ton of 3D conversion calculations on the fly.
You simply can't avoid the deficit. And performance issues like frame-tearing - which are very common in current-gen console titles - simply can't be swept under the rug in 3D. They'll rip your eyeballs apart. So programmers need to keep heaviest load processing in mind at all times in 3D, meaning we're likely to see concessions elsewhere in the graphics, physics, etc.
However, Sony still considers the Wipeout conversion a success. I just can't help to think that HD was already the burden of this generation of consoles. To stack 3D on top - to essentially double the demands of each frame - will prove far too costly for most developers to bother. [Eurogamer via Slashdot]