Just a quick note to those who like to tweak their systems. Prior to Mac OS X I’d use ResEdit and PowerPlant to modify program interfaces, and since OS X Interface Builder has been a tweaker’s playground.

Leopard introduces code signing, which protects against malware even theoretically tampering with an application. Code signatures can be verified on the command line like so:

# codesign -vvv /Applications/Safari.app

Unfortunately, any modifications to files inside the application package will invalidate the application signature. This includes trivial changes to NIB files. The resulting application will still run, but access to the keychain will be restricted, as mentioned in this hint.

In my case I was simply changing key shortcuts, something better handled through System Preferences anyway. But keep this in mind before you start modifying NIBs or other application resources.