Hey Eric,
You're right, the uninstall on Mac is just "delete the *.app file". So, the way to handle it is to have a Activate/Deactivate menu in your app. Then, tell the user to deactivate before uninstalling the app.
Uninstalling on OS X
I'm using both the Windows and OS X versions of TurboActivate. With the Windows version, I noticed that there's an easy way to deactivate the app when uninstalling, as described on the InnoSetup page. However, since there really isn't a concept of "uninstalling" in OS X, I'm not quite sure how to deactivate. Any suggestions?