You are absolutely wrong that Vin MUST to be more than 7V, nothing of the kind.
What happened if you apply voltage less than 6V?
Nothing, read how voltage regulater works, chip will just supply "at most" to the load,
in this circumstances Atmega328 will get 5 - 1 = 4, that is good.
Have you studied the Arduno auto-voltage switching circuit. Tell me how the external power will be utilized by the arduino board if it's at 5vdc and if the board is connected to USB power to upload or monitor serial data. Notice the op-amp circuit on the arduino board, what is it's function and how specificaly does it operate?
http://brittonkerin.com/annotateduino/annotatable_duemilanove.html
Again you stated simply that one could apply a regulated +5vdc to the Vin pin. Is that still your opinion?
There are several ways to power an Arduino board, even applying regulated +5vdc right to the +5vdc pin, however the board is designed to operate a certain way under all conditions such as uploading, running, serial monitoring and being able to supply voltage to external components up to a certain current limit.
My main point was applying regulated +5vdc to the Vin pin is not such a way.
The Arduino 'official' specs for external power voltage is stated as: Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Lefty