growlnotify 1.3 only works with growl 1.3, and is not reverse compatible with growl 1.22.
This is an issue because growl 1.3 is a paid product, and growl 1.2.2 continues to be the freely available version for developers.
Perhaps the best path here is to include both binaries (1.2.2 and 1.3) in the download, and have the install script be smart enough to detect which version of Growl is running on the system? This could enable the growlnotify homebrew recipe to remain a single package?