Atmel is working hard on low voltage MCU, and the latest one is ATtiny43U, it is designed for application running battery. The key feature is the built-in integrated boost convertor to geneate stable 3.0V from voltage source as low as 0.7V. For more informaton: please check out the product web site: AVRtiny43U,
Also, be sure to check out the AVR TV demostration on ATtiny43U:
"Ultra low-voltage AVR with Internal Boost Regulator"