In the microcontroller world, there are three classes of power consumption: the “whatever power”, the “low power”, and the “ultra-low power”. For all intents and purposes, “whatever” and “low” power have converged – you can get a 100MHz 32bit microcontroller (including 12 bit A/D units, 128KiB of flash, a bunch of peripherals, and more) for a mere $5, and it only consumes south of 60mA at full speed (@3.3V). The amount of work done per MHz is amazing, even compared to an 8-bit AVR.
However, there are very few players in the ultra-low power space. Ultra-low power is characterized by very low operational currents coupled with ultra low sleep currents, often with clocks running. Texas Instruments is the former champion here, with their MSP430 line. Its a clean 16-bit architecture (now bizarrely extended to 20-bit for more memory space – flash and RAM is all in one space as most modern processors), where the new parts have run currents of < 200 microamps (uA) per MHz, and sleep currents with 32kHz clocks running at 2.5uA. This is what you need to run for years on AA batteries.
What if you could exceed that, with an ARM Cortex-M3 core, with hardware divide and MAC, running at 32MHz, and a host of nice peripherals?