Really? Is that documented/explained anywhere? I've been wondering how the new power circuit interacted with +5V via the power connector
Technically it works with +5 Volts at the power jack or at the Vin. But the recommended range is from 7 to 12 Volts. The new arduino board uses a low drop voltage regulator. That means the input voltage must be only a little higher as the output voltage (5V).
If you run it with 5 volts it is not in a stable range and you can get troubles with your circuit.
I couldn't find any documentation but I checked the schema how the power supply is implemented.