Nissan has announced that it is increasing the price on the 2012 Leaf, by $2,400 over the 2011 model, to $36,000. Compare that to the price of the Volt, at $40,000. Nissan also is capped at 20,000 units/year for the U.S. market, while GM has said it plans to produce 60,000 Volts in 2012.
If the Leaf is scarce, dealer markups may continue, reducing the price advantage further.
Personally, if I was in the market for a plug-in car, I would probably choose the Volt over the Leaf, buying the gasoline hybrid capability for an extra $4,000 or less.