I don't want to muddy up the issue but shouldn't he shoot high on voltage supply and regulate down? I know that Vregs are very inefficient and maybe i'm just paranoid but i would be leery of plugging my u-controller into an untamed voltage source. Also my paranoia may just stem from me not knowing how stable batteries are or the possible things that could go wrong. Maybe I should be asking that question instead of projecting my paranoia on someone else's project.
I disagree. Batteries are pure DC and fairly stable.
They do decrease as noted by the other poster but this is rarely a problem. I would avoid a regulator if at all possible since they are inefficient and I have lost an FTDI chip from one that has gone short. If this input is less then the chip maximum, it is less chance of blowing things up.