GM figures that it takes 8 kwh to give the Chevy Volt a 40 mile charge. They say at 10 cents per kwh that's 80 cents, so works out 2 cents/mile. More realistically it's 20 cents per kwh, so $1.60, or 4 cents/mile. At $3.00 for 1G gasoline, a 52 mpg Prius is about 6 cents/mile.
EDIT: 4/23/2009. My prius has been getting 44 mpg recently, so at ~$2.00 for 1G gasoline, that's roughly 4.5 cents/mile.