After numerous reports of poor battery performance under OS X Mountain Lion, ArsTechnica performed tests that confirmed a 38% reduction in battery life.
Our own testing revealed similar (and significant) drops in battery runtime after installing Mountain Lion. In previous tests, we were able to regularly achieve just over 8 hours of use by relying solely on our Retina MacBook Pro's integrated Intel HD4000 GPU. Performing the same "real-world" test using the same software applications and usage pattern, we never got the Retina MacBook Pro to run for more than a few minutes past 5 hours after a full charge.
Steve_c, a user contacted by Apple Support about the issue, confirmed it was software based. "I can confirm this is definitely a software (OS) issue, as the guy that was dealing with me said that an update will be issued via the [Mac App Store] as soon as they can work a fix."
Apple hasn't officially confirmed a fix is coming but the site did forward all their test results to the company for analysis.
Let us know if you've experienced an issue with battery life after installing OS X Mountain Lion in the comments.