I've been using a
MK802 II for the past 2 months to mine with my BFL Single. The CPU themselves aren't anything amazing, but they are great for USB FPGAs (and future ASICs). I've heard of a lot of people using a rPi to do the same thing, but it can sometimes have USB issues.
The Avalon ASIC does a similar thing and comes with an included
chipset that runs openWRT to run the mining software, and is also pretty low power (the chipset, not the ASIC
).
So yes, there are all kinds of low power options for using USB FPGAs and eventually ASICs. Almost any low power arm system that is capable of running Linux can be used to compile CGMiner and all it's dependencies. The biggest issues are price, stability, and availability. I consider the MK802 II to be excellent in all 3, but as I said, a lot of people like the rPi.