This is great...but get his name right...and link
It's Johan Kanflo. Link here: http://johan.kanflo.com/the-aaduino/
File this under "why didn't someone think of this before?" Maker Johan Kanflo has cooked up an Arduino board that fits into a AA battery housing. The "AAduino" can also be powered by a single AA, but still packs a pair of temperature sensors and an LED. The image below tells the rest of this story. ® AAduino
I'll tell you why nobody did this before - because the utility is rather limited when one wants to do _anything_ other than measure a temperature and radio it away. The PCB is enclosed in the battery box (and it better be in a closed box because not much else is keeping it there) and any I/O you'd want to do you'd have to wire through that - meaning you might as well keep your PCB next to whatever else you're connecting to. The gimmick of stuffing the thing next to the batteries only has notable advantages if there's nothing else connected...