From the developer:
Hiss is an OSX app that forwards notifications from Growl enabled apps to Notification Center in Mountain Lion.
It’s still abet app, so you can try it for free. The app replaces Growl (looks like you still need Growl installed, just not running) on your Mac and shows the notifications in ML’s Notification Center. It works. It has no preferences for what is shown, which is a bummer, but I do like the consistency of using this instead of Growl. Worth checking out for sure.
Update: Looks like there is a version of Growl that is currently in beta that moves notifications to 10.8 notification center — which will make this app redundant. Still great until then.