Bike share systems come up periodically over a period of years and in many communities (the WikiPedia page has a lot of info). The latest systems are electronic rental systems, with custom bikes and special locations for pick up and drop off. Very simple systems have existed in the past where bikes are just shared in a totally free way -- these systems are extremely cheap to set up, but the bikes always end up trashed or stolen. (Though there's a system in Portland that seems to just use a universal combination.)
I can imagine a very simple system based on combination bike locks, where the combination is sent via SMS, and locations are tracked (somewhat) voluntarily. You would sign up for this service, maybe placing a modest deposit (and also confirming your identity). Then you would send your location to the service (as a text) and it would respond with the nearest bike, and the combination. You would use the bike, and when you were done you would text the new location of the bike.
There would be some accountability with this system, because if you simply don't return the bike there would be some record of this. The service could also continue to pester you if you don't record the new location of the bicycle.
The security is not very high, and it would certainly be possible to take a loss on many bikes. But considering people *almost* considered it economically viable to do this sort of system with no money or accountability involved, perhaps just a small amount of accountability would resolve these problems. The capital expenses should be much smaller than more formal systems.
(I first thought about this idea here and restated for this forum)