[UPDATE: Oct 10th, 2012]
Since I posted this idea in 2011 I have discovered a startup that is trying to do something similar: SILVERCAR (www.silvercar.com). I believe Silvercar was founded in early 2012 as its website in April 26th, 2012 looked like this: http://web.archive.org/web/20120426215703/http://silvercar.com/. Since then they have raised $11.5M from CrunchFund, Dave Morin and Others (http://techcrunch.com/tag/silvercar/). Silvercar is the airport car rental experience reimagined for you, the connected business traveler. They put the power back in your hands, enabling you to bypass lines, avoid complexity and frustration, and drive away in a world-class car at a great price (http://www.crunchbase.com/company/silvercar). @SilverCar is based in Austin, Texas. I hope they do great and revolutionize the (airport) car rental industry. Finally something new in that boring industry!... I cannot wait to see it at LAX and NYC.