Today I got a mail regarding Xcode Templates and it made me look into it again. I was surprised that Apple changed the way Xcode integrates into the system now. Until 10.5 everything was stored in /Library/, but as of 10.5 Xcode uses the folder /Developer/Library/. Everything related to Xcode is there (especially in the Xcode subfolder): the frameworks, plugins, templates… all of it. The plugins that were installed in 10.4 will have to be moved there I guess (got no time to try out if BSInspectors work yet).
So as of 10.5, install custom project templates in /Developer/Library/Xcode/Project Templates/ 🙂