While Notification Center will remove a lot of Growl's requirements, Growl still has advantages that will keep it installed on my, and many others' Macs for a long time.
The Growl team have announced that they will be making a Hiss-like move in adding a Notification Center action display.
For now though, Growl is much more customisable, has an advanced history log, works in many apps (I'm sure Notification Support will grow quickly, but some apps are not still being developed) and has AppleScript support. I'm sure there are other Growl-exclusive features I am forgetting.