I noticed that the Twitter client stopped giving me Growl alerts, so I tried to re-install to see if that helped. In an attempt to completely purge it from my system, the app was removed from the 'registered applications' list in Growl as well. I've removed all related preferences, etc.
I can confirm that Growl actually works, as other applications support it just fine (including the CLI growlnotify).
I've taken to re-installing the Twitter app from the Mac App Store, but that did not help. I did have to reconfigure my alert messages, but after selecting 'Growl' the application did not register itself (and thus does not appear in the Growl preferences).
What am I doing wrong? Or did developers break the Growl support in one of the recent updates?