Vélib, Paris' bicycle sharing system is a year old. The name Vélib is made up from two words — velo (Bicycle) + liberte (Freedom).This excellent Streetfilms video (love the soundtrack used!) takes a look at the success of this network that has 20,600 bikes, and 1,451 docking stations, which are never more than 1,000 feet apart.
Part of the reason for its success is the scale. You can't just have a few locations and a couple of bikes for this to work. You need the critical mass. Another similar project in Rennes, France, was less successful because they did not have enough docking stations and bikes.
It also takes political will and financial commitment to create a network of safe bicycle facilities that is necessary for a bike sharing system to work. Paris has 400km of bike lanes currently (up from 200km of bike lanes in 2001).
In cities like Washington, D.C., they are experimenting with bike sharing too, called SmartBike DC led by giant media company Clear Channel (hey, don't we have Clear Channel here too?). I will definitely try it out and give you guys a report when I am in D.C. in September/October.