A lot of people seem to think that the ARM 32 bit MCUs have some power disadvantage compared to the 8 bit MCUs. I have not seen it myself. The CM3 devices from Luminary Micro are not as low power, but then they are the first generation.
I have looked at the AVRs and many of the other processors and, other than the sleep mode where some individual parts are highly optimized, the SAM7 and LPC2xxx parts are neck and neck with nearly all 8 bit parts. The SAM7 parts are about 0.5 mA/MHz even while running at full speed with parallel I/O, UART, SPI and TWI all running. That is about the same power consumption as the 8 bit parts.
Another thing I consider a red herring is code density. I don't care if one processor uses 40% more Flash space. I just care that I can make my app fit in a given part at a given price. The SAM7 parts are cheaper than the AVR parts at a given Flash size, so even if I have to bump up to the next size flash it is still a break even on price. With the SAM7 parts reaching 512 KB of Flash, I don't see Flash size being a limitation compared to any of the 8 bit parts.
So why consider a 8 bit MCU at all at this point unless there is some peculiar need that the ARMs can't satisfy? For general processing, including power constrained processing apps, the ARMs are pretty hard to beat.