For those of us who have ATI cards they've just released a control panel version that works properly on Powerbooks and new iMacs. Out of curiosity I tried setting Second Life to high speed...
Performance = "speed"
Anistropic Filtering = "Off"
Vertical Sync = "Off"
I keep FSAA on because it's so much nicer.
The effect is a noticeable FPS increase.
Not sure what it's actually doing, if I had to guess I'd say it's adjusted texture mipmapping so that the high res textures are only displayed when you're very close to them. I've noticed distant items are more blurred than usual but closer items are ok.
Personally I like lots of detail even at the expense of FPS, so I'm not sure how long I'll use this option, but it's worth a mention.
you can find it here