Growl's response to Notification Center
growl.posterous.com
growl.posterous.com
(1) Seamlessly integrate growl into iOS notification center, function more as a transport layer and delegate UI to Notification center if present. Use it's own UI if not. (2) Allow growl to be paid for by the end user as a standalone app (3) Allow a limited version of growl to be bundled with apps with a discounted rate for app developers. This version would be limited to notifications for that app only, but would only cost that developer something like $0.10 per license.
Basically adopt a strategy that doesn't require users to have to buy and install Growl to get notifications in non AppStore apps. Let the developers who want notification support choose to handle distribution of growl inside their app and pay the license cost. Notifications are such an important part of engagement, that I'm sure there is a market opportunity in charging the app developers instead of end users.
At least that seemed to have been the case with apps using Growl because I still see notifications but don't have Growl installed since they went paid.
I was under the impression that a user on Lion or later, would not have access to growl without paying for it. I'm specifically talking about the use case, where one of your users isn't already a savvy current growl user, but just a regular joe that is unlikely to have ever installed growl. In other words, the use case where the kind of person using your app will miss out on notifications because they don't know better and won't install a third party notification tool.
A gentle reminder is good enough, and might induce me to pay for the new paid version. However, if you take an app which used to be free, and then nag me constantly because you decided to make a non-free version of it one fine day is annoying.