I searched the net for examples to follow in my build and found this nice one: http://www.boxwave.com/products/batteryadapter/ and this cheap one: http://www.dealextreme.com/details.dx/sku.15048
Much better, IMO, than the $20 Radio Shack one that's only 650mah: http://www.radioshack.com/product/in...&tab=techSpecs
NiMH AA's peak at about 1.4V and are done around 1.2V. So we're looking at between 4.8V and 5.6V for the pack. No voltage regulation is needed, unlike the 9V-to-USB adapters floating around (which have very little power to offer).