For RR and other technically inclined among us:
http://gm-volt.com/2012/04/13/cost-per-mile-comparison-2012-volt-vs-2013-prius-plug-in/
Are we talking 3 year leasing or long term ownership

The Volt is an experimental vehicle. The only way GM can move these units is by offering a ultra ridiculously low 3 year, low mileage, lease with bumper to bumper coverage.
To access the battery in a Volt, 600 parts need to be removed not excluding the seats and doors.
As you pointed out the Prius Plugin is a Prius with a different battery. The Prius has been Toyota's most reliable car. The Prius Plugin is not an experiment.
I think fuel consumption comparisons are irrelevant between these two cars because it will be insignificant when the long term cost of ownership is factored in.
If you want to compare a current low mileage 3 year lease, the Volt bests the Prius regardless of running costs providing you can wait 10 hours for a recharge.