Had to bring this one back up, as I saw it searching around.
The reason for the battery location (as said directly by a GM Lambda engineer in a video) is twofold:
1) Lends itself to better weight balance. Granted, this isn't a light sports coupe, but every little bit helps, and taking the relatively large weight of a battery off the nose and putting it in the middle is beneficial on a small scale.
2) Being out of the engine compartment, and essentially interior mounted, it is taken away from the high heat, dampness, and dirt that invade the engine compartment over time and actually reduce the battery life. It also further cleans up the engine compartment, leaving more space.
There's quite a few cars that have the battery in the trunk or under the rear seat, and it's for the same reasons.