keyhole frustrum?

According to Vektor Lindem's preview FAQ textured object in Morris near 231,131,89 (east side in the middle), it says:

Where is this option? I can't find it anywhere and don't remember EVER seeing it in 1.9.1 preview. It actually seems like SL's been doing that ALL the time since, at least from the mini-map and the ground, anyway, it doesn't render until it's in the camera's view.

Maybe it means in relation to draw distance? I've lowered my draw distance in 1.9.0 to 64 metres, if I place my avatar at the end of a 90m long room, I can't see the far end of the room (the objects simply aren't there beyond 64m, as expected). If I use alt to zoom my camera to the far end of the room, then I still can't see anything, but if I look back I can still see everything within 64 metres of my avatar ('in a ring').
Try it with an even shorter draw distance and you should be able to see what I mean anywhere.

It must mean then that rendering is now performed from the camera's position opposed to the avatar's, so if I now move my camera over 64 metres away from my avatar then I will be unable to see myself, or at least the objects around me.

I can't check though as it won't let me into preview atm

No, that's not it; that's how SL's has always rendered (for me anyway since 12/04)--in a radius around the avatar (not the camera). The only thing that doesn't seem to render radially is terrain. None of this has changed in SL 1.9.1.

And, incidentally, the draw distance can't be set lower than 64m, unfortunately.

Hmm, I can't double check this one, as SL no longer seems to be respecting draw distance or occlussion culling at all for me =S

1.9.1.25 respects draw distance for me, but OC is foobar.