You might find it useful to take a look at the theme_support plugin (http://www.agilewebdevelopment.com/plugins/theme_support) to see an example of how to extend the places that Rails will search for layouts and views. From that starting point, it shouldn’t be hard to come up with a shared path you could use for your various apps.
An alternative approach if you are using subversion would be to use svn:externals to share paths like app/views/layouts and public/stylesheets. This approach would probably be more straightforward.
