I picked this little developemt board from coolcomponents.co.uk, the A-Star 32U4 Mini Ultra Low Voltage (ULV). It combines an ATmega32U4 chip with a step up regulator (TPS61202) to boost battery voltages up to 5v. It has a small form factor and the boost circuit makes it ideal for battery power projects so it can be used on and off the bench. There is a little caveat here, I am not sure if the advertised 0.5v-5.5v is all that accurate. It could be that the batteries I used were not good enough but someone more used to testing power regulation would be a better judge.
If you fancy picking one up, they are only about £15 and I think they are worth it – http://www.coolcomponents.co.uk/a-star-32u4-mini-ulv.html
For details from the manufacturer – http://www.pololu.com/product/3102
And yes, I was hungover.