According to their tests, the new 15″ MBP lasted almost TWICE as long as the previous model– a total of about 5.5 hours to a charge. That is a huge jump, and as far as I can see it is unprecedented. They say:

There’s only a 46% increase in battery capacity, there shouldn’t be a ~100% increase in battery life…ever.

I figured Apple must’ve had a damn good reason for making the battery internal, and it looks like this is it.