The GM insiders said they relocated it there for better weight distribution, as well as moving it away from the heat of the engine lengthens battery life. There are still positive and negative posts under the hood for jump starts. Now that's good engineering!
My 2004 Volvo has the battery in the trunk area under the floor. Actually a very good idea. It's easy to get to and you can still jump start from under the hood. There was a big scare for about a year with people saying the emf radiating from the long cable under the floorboard was harmful to passengers. It finally died away and it looks like we're all still alive.