Gotta Love Growl
I've been running Growl for a while now on several of my Mac laptops (PowerBooks then MacBook Pro). It's a beautiful little service that I have always thought would have great uses, but it was going to take developers to get on board with it to make use of it. There was no way to put the hooks in after the fact. Thankfully, it seems that Growl has achieved a critical mass and is looking to be a 'necessary feature' in a lot of the better Mac software these days.
About the only apps I use that don't support it are the Apple apps (Mail, Safari, Xcode) and the apps where it makes no sense to support it (Acorn, BBEdit, etc.). I'm not sure that there will ever be support for it from Apple, it seems to put a reliance - allbeit a weak one, on the Apple apps. But certainly they should consider it for Xcode and other things that could really use the notification capabilities.
Recently, the Growl developers released 1.1 and then 1.1.1 of Growl and I have been a faithful updater. I didn't get into a lot of the problems that the changes in these version addressed, but I can imagine that I might have if I were using just a few more Growl-enabled apps. Growl, like Sparkle, is one of those things that were I to make an app for Mac OS X, I'd certainly put in the support for it. There's just no reason not to - it's just too neat.