I took a look at the Growl support for OS X. It's not really a proper implementation, as it relies on the debug log functionality instead of creating a plugin for Growl. If anyone else has started code somewhere, it'd be good to collaborate.

For inspiration, here's a screencap of the Boxcar plugin. It has pretty much all of the settings I need:

http://skitch.com/deviantintegral/d799e/growl