Toyota says not to tow anything with a Prius.

The Prius is not designed for towing.  The owner's manual says not to
tow, and the warranty documents tend to say that warranty is void if
you do not follow the owner's manual...

However, there is a Toyota-approved fitted tow bar available from
Toyota in the Netherlands.

Some people have installed a receiver hitch, though.  Some examples:
http://john1701a.com/prius/prius-hitch.htm
http://www.coastaletech.com/hitch.htm
http://www.coastaletech.com/04hitch.htm

Even still, a few people do use their Prius for some very light towing
of small trailers (like those for a motorcycle), so it is not
impossible, but you'd have to be careful of the weight limits of the
vehicle and drive gently...
http://autos.groups.yahoo.com/group/priustrailers/

I have an NHW11 Prius with a 2" receiver hitch. I put together a
light-weight, folding trailer from Harbor freight to handle oversized
loads. I've also towed a flat-bottom fishing boat. The only problem with
the boat was blowing the tail light fuse. However, I have extra
instrumentation.

I have a Graham scanner so I can monitor temperatures and a thermistor
in the transaxle oil pan. However, I don't plan to do any high-speed,
cross country towing. I typically tow at city speeds and fairly flat
areas.

Bob Wilson