According to Wikipedia, there are about 200 bicycle sharing systems worldwide. Fewer than 10% of those are in the United States, and one of those is the B-cycle program in Denver.
B-cycle is actually a multi-city program, with installations in six other US cities (including San Antonio). We checked out the Denver installation today and found it to be an impressive service, but highly dependent on the having the right infrastructure.
The concept is simple: check out a bicycle for a nominal fee (which is charged to a credit or debit card) and use the bike for short trips throughout the service area. If you use the bike for trips of less than 30 minutes, you're not charged any additional fees; longer usage times incur increasingly expensive fees. The idea is to keep people from tying up the bikes for long periods, thus making them unavailable to others.
You can buy a 24-hour pass, good for unlimited rides of 30 minutes or less, for $6.00. Residents can purchase memberships that provide more access, and also provide automatic tracking of mileage, average speed, time ridden, etc., thanks to the GPS and RFID technology built into the bikes and the checkout stations.
Denver has 500 bikes in the program, with 50 check-out stations scattered mostly around downtown and in the most popular retail districts that are accessible via the city's amazing network of bike trails.
And it's those bike trails, as well as a general overall bike-friendly philosophy that make the B-cycle concept successful. It's one thing to have access to the bicycles themselves; it's quite another to have a safe and enjoyable environment for using them.
Denver has a quite laid-back attitude toward cyclists. For example, although cycling on downtown sidewalks is technically discouraged, as long as you're not out of control, nobody really cares. Cars give cyclists the benefit of the doubt, a refreshing change from the often hostile attitudes we encounter in West Texas. And, as I mentioned previously, Denver's system of dedicated bike trails, and clearly marked, wide bike lanes make it possible to get almost anywhere by bicycle without competing with auto traffic.
Thus, while such a program sounds attractive for any city, it would be less so in practice than in theory for most locations. A successful bike sharing program first requires a culture of bicycle acceptance (or, better, encouragement), followed by creation of an infrastructure to support the program. For many (most?) cities in the US, I suspect this is never going to happen. More's the pity.
If you ever find yourself in downtown Denver for several days, I highly recommend trying out the B-cycle system. It's a great way to get around the area without worrying about driving or parking. The bikes are well-maintained and easy to ride, even for an inexperienced cyclist.