Tobster86:
Just to be 100% clear; will the Arduino function as normal from 5V by simply supplying 5V to it's 5V pin and leaving the Vin pin disconnected?
The official Arduino page says "no": http://arduino.cc/en/Main/ArduinoBoardUno
...but nobody can figure out why. The only difference between USB power in and the 5V pin is a fuse so the usual conclusion when this subject comes up is that they're just covering their asses because the 5V pin has no protection whatsoever.
Here's the schematic: http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
The fuse is "F1" on the "XUSB" line at bottom-left.
I've done it many times and nothing bad ever happened but if you're truly paranoid, power it via a USB cable...
PS: I prefer to connect 5V power to the power pins on the ISP connector, but that's just me. You can even solder them on, just to be on the safe side (use the connecter up near the USB socket if you do this - you're unlikely to ever use that for anything else).
More discussion here: Arduino Uno R3: Directly supply regulated 5V to 5V pin? - Electrical Engineering Stack Exchange