Which technology will revolutionize hotel room keys?

Hotels are already equipping their doors for the future — Examining technologies that can bring "non-stop check-in" to hotels at Hospitality Net.

The contenders:

1. Smartphone with app 
2. Traditional mobile telephone 
3. Universally programmable key (includes certain advanced house, office or car keys) 
4. PIN code 
5. 2-D barcode 
6. Fingers, hands or eyes (guests tend not to leave these items at home)

Author Keith Gruen breaks down the pros and cons.