I have recently been trying to figure out how to accomplish this same thing. My clients are only connecting from Pidgin and Adium. But people are figuring out they can use mobile IM clients like IM+, and Trilllian to login. They tend to forget that its logged in and messages are only sent to one resource or the other, so they are missing messages. There might also be a bug ont the mobile side after killing the app the account does not get disconnected. I have the server set to kick logins on the same resourse, but im trying to restrict the same username from logging in more then 1 time.
-Aaron