Well that is actually one of my problems with the Prius, you can't use it for towing. I'm not going to tow a boat or any such silly thing, but sometimes when you need lots of cargo space for helping someone move f.inst. it is usefull. In my opinion a towing capacity of 500lbs is decent and 1000lbs would be perfect. That would closely match most normal family cars towing capacity here, like the Ford Mondeo, VW Passat, Volvo V70 etc.
I've got the feeling that is a difference between Europe and US, where towing capacity there is measured in thousands of pounds while in Europe it's usually a question of wether it has a hitch or not.
Cobos