THEY DON'T DO THAT. They're polarized.
LCD shutter glasses are completely different - actually, they're more like really clunky goggles. Here's a picture of one:
And there's no light loss as a result of using them - one eye always sees a fully-lit frame. Darkness would only result from both lenses being switched on and off at the same time, but that would kill the 3D effect anyway.
You know, now that I think about it, I'm not sure that it's really all that feasible. I was thinking that the PS3 would be able to simply render 3D graphics from one view for a single frame, then from a different view for the consecutive frame, etc, but the games themselves wouldn't allow for that constant "jiggling" of the camera.
Still, it's a feature that could easily be implemented further down the road.
@ Lance
Please stop screwing around with my posts.