Remember that most Mac apps don't have an installer - they're self-contained, and installing consists of extracting the DMG and putting the app on your hard drive.
The notification will probably take the same form as the error message you get upon trying to launch a PowerPC app on 10.6 without Rosetta installed.
TO be fair, even apps without an installer will open a warning dialogue the first time it's actually run letting the user know this is an application that's "new" and hasn't been run before.
Also - as long as Apple makes it easy for developers to get signing keys, which seems to be their direction as far as OS X goes, there's no reason for developers to complain about it too much. Just sign your builds, and you probably avoid any warning message.
The notification will probably take the same form as the error message you get upon trying to launch a PowerPC app on 10.6 without Rosetta installed.