In almost two years of being a ZipCar client, I've never had any issues. Yes, the occasional coke can or candy wrapper left behind, but never a mess that I thought to be unreasonable.
If it turns out to be a bigger problem, install a camera that takes a shot when you take possession of the car, and when you relinquish it. Charge the renters credit card for messiness.
Such a system would be good to have anyway as part of a dashcam. One facing forward, and a fish-eye facing the rear window and also capturing the interior of the car. I am hoping that car insurance companies will start offering discounts for cars with installed dashcams.
A tangent of a tangent now, this would be great (though creepy) for public restrooms. Given that there's a physical method, like positioning on the door, that prohibits it from seeing anything while the stall door is closed or locked, I think it would cut down on the seriously depraved messes people leave behind when they know they can get away with it.