When you would want to plot (or most other commands) without a dialog box you would use "-" in front of the command. For example typing in "-array" at the command prompt would start the command without the dialog box.
However your problem is the FILEDIA setting. The following is right from the AutoCAD 2004 help file:
FILEDIA controls the display of dialog boxes used with commands that read and write files. For example, if FILEDIA is set to 1, SAVEAS displays the Save Drawing As dialog box. If FILEDIA is set to 0, SAVEAS displays prompts on the command line. The procedures in this documentation assume that FILEDIA is set to 1. Even when FILEDIA is set to 0, you can display a file dialog box by entering a tilde (~) at the first prompt.
FILEDIA and EXPERT are useful when you use scripts to run commands
Hope this answers your question. If you have any more just shoot me an email at jstanco@eypae.com. If I can't answer it I work with a CAD programmer that can.