The $1500 sounds like the holdback the dealer gets when they sell the cars for which they have been invoiced ($1200 plus $300 for the trouble). In any way, the Volts seem to be brand new, "zero"-miles cars in perfectly good re-sellable condition
I do not know of other demos, but my Nissan dealership has a demo car that arrived with over 11 thousand miles in the ODO; not what I would call a "new" car.
Maybe the LEAF demos are the cars that were put through months of testing before the first deliveries?