How does it work? Well, there are B-cycle stations all around Denver. Once you have an account, you can grab a bike from any of them. When you're done, you can then drop it off at any other station. The GPS unit inside the bike (there's no nav screen, it's hidden inside the bike itself) tracks your route and lets you check it out online when you're signed in.
It's a pretty awesome idea, with custom bikes made just for the system by Trek. It's pretty reasonably priced, too, with a single day costing $5 and a year costing $65, plus daily hourly usage fees.
They're currently finishing the details on the next batch of cities, although they're mum on what those cities will be. You can vote for your city on their site if you want to try to get it near you.
(I was told that in order to bring the system to NYC, they need a whole lot of grassroots support and people asking the city for this. Selfishly, I want that to happen. So start harassing Bloomberg about it! Let's make this happen!) [B-cycle]