They found that Apple squeezed a 10-hour, 65 watt-hours lithium-polymer battery into the case. While it is labeled with the same model number (A1331) as the previous generation of 10-hour batteries, this generation weighs 7.5 grams more and packs an additional five watt-hours (up from last generation's 60 watt-hours).
iFixit tried the new battery in the unibody MacBook and found that the newer battery does, in fact, work. They note that if you're willing to swap batteries and add a few extra grams of heft, you can add 350 extra mAh's of battery life.
With this finding, we're wondering if Apple will be placing these new batteries in the next iterations of MacBooks and MacBook Pros. You can read more about the tear down on iFixit's website.