hi
i've done a couple of shoots with a 5D2 tethered via a 5 metre nikkai usb2 active extender to a quad core intel mac on OS10.4. shooting raw plus medium jpegs, the jpeg seems to arrive in a blink, then the raw appears after another second or so. we did a couple of shots with two of them (so 10 metres plus the maybe 2 metres more usb to the camera) and that worked too, live view and all.
[attachment=11371:5D2_0155aLc.jpg]
it's slow shooting of maybe sixty files though so i can't comment on the usb slowdown experienced by fast portrait / fashion photographers.
using canon's software to do the tethering thing and control the camera, and a cs3 bridge window open (filtered to just show jpegs) each new file just pops up. all quick and easy. i can't claim that this is the best or most efficient as i haven't had the time to play around, it's just nearest what i'm used to and was a quick solution. hth.
pic is two exposures blended, shooting tethered suits this sort of thing.