In Germany we have a system like this called call-a-bike (it's run by the german railway!). It works surprisingly well. You're allowed to leave the bikes wherever you want and if they are left unused for a while a truck will come and move them back to a central location.

I really like what via cycle has done with the SMS unlocking. Back when I was in Germany, almost three years ago now (!), you had to call a service line to find/unlock a bike. Late at night there were usually wait times and it was especially difficult to manage the entire process if you were a little tipsy.

There's similar systems in many other European countries. I hear great things about a system in France called velib (I think).

Edit: I forgot to mention, the German bikes also have a second seat on the back. I'm not sure from the picture if this is also true for viaCycle but it's definitely one of the best features. Here's a picture http://www.pedelecforum.de/forum/imgcache/5382.png*