Growl's lead developer Chris Forsythe explained in a blog post that Growl will be embracing Notification Center rather than trying to compete with it or shutting the project down. Part of the reason for this is that Notification Center will only be accessible by apps sold through the Mac App Store -- standalone apps offered through a developer website won't be able to access the Notification APIs.
Writes Federico Viticci for MacStories:
The introduction of Notification Center won’t necessarily “sherlock” Growl as a notification alternative for Mac users and developers. While Notification Center will undoubtedly provide an excellent native solution for developers of Mac App Store apps — ultimately serving the average user with a notification system that works out of the box, — Growl will still remain a capable third-party app that offers fine grained controls over notifications with several customization options.One particularly useful new feature, called 'Mist', that Growl is planning revolves around making life easier for developers. The Mist API allows developers to implement one piece of code that will send notifications to Growl, if it's available, and Notification Center if it isn't. Growl has delivered one piece of code to support both its own notifications and Apple's Notification Center.
Growl developer Chris Forsythe:
The one thing that has kept me up at night since the 10.8 release was announced, is that I really want to be able to help developers transition from Growl to Notification Center on their terms. We could have taken an aggressive stance and said "No, Growl is more awesome!", but at the end of the day that isn't us.Growl is available for $1.99 on the Mac App Store. [Direct Link]
Here's the important takeaway. I said this same exact sentence 73 times. I'm going to say it 73 times more today. "It would be irresponsible for us to not support NC and abandon our developers, so we're going to embrace it." And that's just what we're going to do.