Someone had mentioned earlier in this thread that voltage was linked to clock frequency, so thats what I was going off.
Couldn't spot that statement just now, but... looking at the datasheet for the ATmega328P it does say that it will run at:
1.8-5.5V for a clock of 0-4MHz,
2.7-5.5V for clock 0-10MHz and
4.5-5.5V for clock 0-20MHz
So running at 4MHz in principle should mean you can go down to 1.8V which would give you decent battery life on AAs I guess.
Trouble is that if you're using the clock tweaks described previously then you'll still need to boot up at full clock speed first. Hmmm!
(You would probably need to connect Vcc on the pro direct to your supply as well, rather than going through U2, whatever that is - presumably a regulator.)
Sounds like it would be easier to reprogram a 4MHz bootloader into the 328, really.
I'd be interested to know how you get on.