What makes SSHD different than other 3D experiences -- specifically, Avatar and other 3D games like Invincible Tiger -- is that it'll be running at a native resolution of 720p at 120 frames per second, 60 frames per each eye, and doesn't use low-res buffers to create the effect. Housemarque accomplished this by having the engine render everything twice, added stereoscopic cameras and moved the vertex processing from the GPU to the SPU. This took up about 50 percent of the SPU's resources, leaving them with enough to pre-process a lot of the game, allowing things to render much faster than before.
As for future Housemarque games on PS3, don't necessarily expect those titles to be in 3D. Tikkanen says that because some titles "lend themselves better to 3D than others," all future 3D compatibility will be decided "on a game by game basis."