In the 2.0 version of the Growl SDK, we introduce Notification Center support. If Growl is not running, the app will post directly to Notification Center. If Growl is running, it's assumed the user wants Growl, and the app will post to Growl.
Here's the fun part.
In Growl 2 there will be a big switch that you can turn to on, to turn on NC support in Growl 2. The way it works is that any app that has the 2.0 version of the framework will be notified that the switch is enabled. Those apps will send to both Growl and Notification Center.
If this switch in Growl is enabled, then the visual notifications portion of Growl will be grayed out and you cannot use them. However, actions will work. So you can send notifications to Prowl/Boxcar/MailMe/etc etc and have Notification Center be the visual notifications.