http://www.trittontechnologies.com/products/TRIUV100.htm
Tritton SEE-2 USB 2.0 video adaptor -- This is PC only right now, although they are working on Mac drivers.
The bandwidth needed for display is dependant on the resolution, the color depth and the refresh rate. At 800 x 600 with 24 bit color or 1024 x 768 16-bit color, USB 2.0 can keep up.
Programs like games that utilize graphic processing units for acceleration, programmable shaders and on-card memory won't be able to exploit that through USB. So games and streaming video are probably out of contention. But 2-D applications should be fine.
We'll have to wait and see whether they come up with the Mac drivers.
Thanks
Trevor
CanadaRAM.com