Whenever I hire a car or van now, I take plenty of pictures, and then a few more (especially of the corners) before leaving the hire place. I then take plenty more before handing the keys back in. Takes about five minutes, and if they can see you doing this, it tends to put 'em off pulling a fast one.
+1 - mate and i dropped a car back at Dublin airport earlier this year.
They claimed damage.
We said not and refused authorisation to take any further payment.
They said they'd pursue it and brought out a little digital camera to take pictures 'for evidence'.
As 2 x pro-photographers, returning from shooting a wedding we pulled our camera kit out and proceeded to take much more detailed shots - they soon gave up 
seriously though al, i'd check your policy for the 'what you should do in the event the office is closed on returning the vehicle' clauses - if you're within their terms then you may have a leg to stand on. Has to worth a call to your visa card people as well - gotta guess they get this a lot so, they'll have a plan, might not be one you'll be happy with though...
