The Prius and Ford Escape are what they call "full hybrids," which means that at low speeds and certain other conditions they run only on the electric motor. I expect that only this kind of hybrid would get significant savings from using an external source to charge the battery.

The Prius uses a Nickel Metal-Hydride battery (NiMH) which is prissier about charging and discharging than lead-acid types, but doesn't suck nearly as bad in that department as Nickel-Cadmium batteries, and is less toxic.

I suspect that Toyota engineers could outfit a Prius quite nicely and cheaply to accept and regulate outside power, and I wonder if it isn't going to be an option soon.

Interestingly, as I predicted a while back, the battery technology in hybrids has already improved a great deal in just the past couple of years, becoming better and cheaper.