Growl is one of my favorite things about the Mac platform -- it's a notification service whose customization and power is unmatched on anything I've seen in other operating systems. And it's going through some changes in the near future: Project Lead Christopher Forsythe has posted on the Growl Google Group that as of version 1.3, Growl will move from a notification service into a full app, mainly to be included in the actual Mac App Store.
That has quite a few ramifications, both good and bad. The first is that Growl will be able to take advantage of everything the App Store offers, including ease of installation and upgrades, as well as discovery and any future features planned for Mac App Store apps. But the flip side is that anything the Mac App Store doesn't allow obviously won't be allowed in Growl, so those services (which currently include GrowlMail and GrowlSafari) will be retired completely.
This also means that Growl will become an app only for OS 10.7+, so 10.6 (Snow Leopard, the current version) and earlier will not be officially supported. Forsythe says the Lion upgrade is relatively cheap, and it will help the app more to move forward with Apple's OS rather than have to deal with customers who don't upgrade.
There's a lot to deal with here, and there will certainly be more issues and questions that come out as the new version enters the Mac App Store along with Lion. He also says that "these are not all of the changes coming," so don't waste all your shock right away on this one. I love Growl and plan to keep using it through the Mac App Store after Lion's release, but a lot of users who might depend on it for specific and edge case scenarios might be left in the lurch. We'll have to wait and see.