I'm not one to promote apps, but there's app called Hiss that has been released as a free public beta. It automatically listens for notifications intended for growl's API's and reroutes the messages to Mountain Lion's native notification center. I've been using it myself for a few days, and although it's somewhat of a sparse tool, it does the job.
http://collect3.com.au/hiss/
http://collect3.com.au/hiss/