Where to connect shield in USB power supply for Arduino?
I think most arduinos have the shield connected to USB ground anyways. The rules of the USB spec regarding power, grounding, isolation are often broken - either out of ignorance, cost or laziness. It only becomes a real issue when circuit ground and chassis ground are not the same, which can happen with mains-powered devices.

Quick note: if you are only going to use this for an arduino-application, you can just connect whatever voltage you are regulating directly to the arduino Vin pin. It has it's own local 5V regulator. Of course, using a USB cable might be easier.