Agreed. If the bike doesn't get returned, simply charge the account for the cost of a new bike.
It would also be helpful (and maybe costly) to implement some RFID solution to keep track of the unique bikes and the user pairs. If a bike is damaged, the rider can fill out a form online and the company can charge-back the previous user if its not standard wear and tear. This would cut down on the vandalism of the bikes.
Agreed with make "create an account more distinct. Possibly a "Sign up" like http://basecamphq.com/signup
Also the CAPTCHA widget sits on top of the textfields in Firefox.