....pretty conflicting statements as to the feasibility of powering an ATMega directly from USB...
I am in the midst of planning my own shield/board that will need to be powered strait from a USB header off of a PC Motherboard
There is nothing conflicting about the feasibility of USB power. There is either enough of it, or there isn't.
The USB supply is regulated anyway. So that takes care of the volts, and all you have to worry about is the current.
If you intend to use just a few low-power peripherals, you will probably be fine, so I won't ask about the logic behind your intention.
Just be aware that, when I first encountered a power problem, it was not because I had done anything to the Arduino, it was because I had changed the motherboard in the PC.