There's a guy on the yahoo prius group that took his off road (through
meadows, fire trails, etc) when pursuing his hobby of photography.
He reported no problems.

The clearance is roughly the same as the Camry.  I seldom take either off
road because of rocks chipping the paint.

Daniel

I live on a gravel road and I've found that stones hitting the underside
of the vehicle generate considerably more noise (compared to the current
model Camry that I also drive) due to the low ground clearance and
aerodynamic covers. Driving up loose gravel hills (even slowly) causes
the vehicle stability system to activate repeatedly. From my experience
I wouldn't hesitate at taking the Prius off road but a Camry would
certainly get further if the wheel tracks are dug in.
I'm not impressed with the low front entry angle of the vehicle since
the car is more likely to scrape the ground on entering steep driveways
compared to similarly sized vehicles. I found that surprising
considering  how close the front wheels are to the front of the vehicle.
The suspension handles pot holes and speed bumps at high speed very well
though.