It appears that NVIDIA SLI owners cannot run the recently released Prey demo. NVIDIA has issued a statement with a workaround as well as word on an upcoming driver patch:
A few enthusiasts have commented that the recent Prey demo does not support NVIDIA SLI. NVIDIA is currently testing a Prey SLI profile, which we expect to release later this week in a new driver on nZone.com.
In the meantime, users can simply copy and paste the Quake 4 profile to make Prey scale. Prey and Quake 4 were developed with the DOOM 3 (OpenGL) engine, and SLI scales very well on Quake 4.
NVIDIA recommends users download the new 91.31 Forceware drivers for Prey. This driver works great on the Prey demo with all NVIDIA GPUs.