If you'd like to extend Metasploit in some way (and it isn't polished enough or applicable to all users to be put into the main framework code), a plugin is probably the best way to go. Many examples in the framework show how to add console commands, but if you want GUI integration, or integration with scripts in other languages it is easier to have RPC methods. If you would like to add RPC methods to your plugin, you can check out this example: http://pastebin.com/2UabJgL6 or http://www.scriptjunkie.us/files/rpcexample.rb. And if you would like support for your favorite plugins (token_hunter? Nexpose? Nessus? Lab?) leave me a comment or send an email.