No, code signing is definitely NOT innocent on 10.5. Mac OS X will actually modify programs if you've enabled the application-level control of the firewall. Thus it will modify all online games. If the service you're using to play the game online checks your application to make sure you're not cheating, it will always fail because the program has been modified.

For example, we'll take a look at Starcraft. If you enable the firewall and set it to allow Starcraft to have incoming connections, Mac OS X will append some junk (obviously not junk, but looks like that to me) to the file "Starcraft (Carbon)". The first time you attempt to connect to Battle.net after installing or updating Starcraft, Mac OS X will prompt you to cancel or allow. Bothersome, given the fact that Starcraft is full-screen usually, but not a problem compared to what happens next. Because Battle.net has already checked your program, you will be allowed to connect and play games. However, the application has already been modified, and subsequent attempts to connect to Battle.net will return an "unidentified application" error. After discovering this I turned off my firewall and stuck to Little Snitch for my filtering needs.

In other words, code signing is not inherently neutral the way it's implemented Mac OS X; it's actually inherently negative.