Have Apps with Notification Center Support Use Growl on Lion
For Mountain Lion

Hiss goes the other way (sends Growl notifications to the notification centre in Mountain Lion).

Growl 2.0 is on it's way and they've developed a new piece of code they call MIST which will send notifications:


to Growl if it's available; or 
to notification centre if you don't have Growl.


This means if app developers use this code instead then people still on lion won't be unsupported.
Read more here http://www.macrumors.com/2012/06/13/next-version-of-growl-to-add-notification-center-support/

Update to ML when you can, it's awesome.

For Lion

I don't think there's anything (right now at least) to turn notification centre alerts into Growl alerts.