There are quite a few cycle hire alleycats around the world. The London ones are fun.
One of my friends rode the Dunwich Dynamo on a cycle hire bike. 200km overnight on a bike that you couldn't service or fix a puncture on... pretty brave.
By the way, stashing bikes behind locked doors is actually what the "OV-fiets" plan in The Netherlands is doing (they are more like lockers, really: http://3.bp.blogspot.com/-PeUfIZbrzY4/Ti8hHZCsLyI/AAAAAAAAAE...), and it works quite well. As a bonus, you can also rent those lockers for your own bike.
http://www.bromptondock.co.uk/
Then you get one of the best folding bikes, perfectly good for general use too.
They have plans for long-term hires too so you can use a bike daily and guarantee it is there for your commute. In the UK these Brompton Docks are mostly located at rail stations so that you have a bike waiting for you at the end.
I agree that there is room for creativity here. There are so many things to try. For instance, I noticed that ViaCycle seems to be piloting on college campuses. Might it be possible to put docking stations _inside_ of academic buildings, requiring a college ID swipe to get in, and possibly being within viewing distance of a building attendant?