the AgentCallbackLogin Application is greatly depreciated in Asterisk 1.4 and the suggested method to create dynamic Agent Login with a Callback extension is to use the new Asterisk Extension Language "extension.ael" file type in the /etc/asterisk file. I've created a sample.
Personally I think it is better the new way, because creating Macro's for extensions. It's a work in progress, but the app. is completed to do use the "AddQueueMember" application and then obviously to delete that Queue Member as well as to pause and un-pause that Queue member. I'm in the middle of getting the App to provide the right CDR reporting variables to be able to report more dynamically the agent specific activity for more of a Wallboard app.