Notification Center is a great new feature in Mountain Lion giving you a quick way to see all the notifications that come into the computer but unfortunately not every application currently supports it.
There are still many apps that use Growl notifications instead, resulting in an inconsistent user experience.
If this is an issue for you, then help is at hand.
Hiss, a beta bit of freeware available here at collect3, will take a Growl notification and forward this to Notification Center automatically
Please be aware that as this is beta software install at your own risk! I have been using it for several months now with no issues.