On my Motorola 2 way radios I can either use AA batteries, OR the 3.6V NiMH/NiCD pack that Motorola sells. When I got the radios, the cradle and one battery pack each came with the bundle, but when they died, I switched to NiMH rechargeable AAs.
Maybe Nintendo could do this - allow the controller to use both AAs and a rechargeable pack, which, along with the cradle, could be sold separately.