SLI is not an option for video editing with Premiere CS5 utilizing the Mercury Playback Engine. In other words, Adobe and Nvidia did not engineer the Mercury Playback Engine to work with more than one card.
Related to your other question, my thought is, Nvidia and Adobe certified a few select cards to work. Rather than going to the expense testing other cards that should work just as well, a hack appeared from who knows where. As far as I know all of the new high-end Nvidia cards work great with Premiere CS5. The hack as it's called, is really a very simple manuver that get's your card recognized in Premiere. No big deal.
Why are the other NVIDIA cards like
No one knows. Given the simplicity of getting them to work, and the large number of people using them successfully, I just don't trust any explanation offered by Adobe as being the actual reason they're not 'officially' supported.
The guys at Studio One did some tests and wrote this interesting article:
It seems that the RAM speed (DDR5 v/s DDR3) is more relevant than any other factor...
The GTX 470 has more cuda cores than a lot of the more expensive cards... go figure?!