I used to hack at apps on my Mac all the time (change the icons/sounds etc... in the resources folder of the package). I have tried with apps downloaded from the App Store, but it no longer seems to be possible. In fact, even creating one empty file will stop the app from launching. I assume that Apple are doing some kind of one way hash of the application folder, and checking if it has changed.
Is it possible to either bypass the code-signing check or be more sophisticated about modifying apps that are signed and downloaded from the Apple Mac App Store?