The whole notion of 'logging in' can be a bit fuzzy. You might be able to accomplish what you want simply by putting the name of the program you want users to run in the password file as the 'shell'. The typical default in Linux is '/bin/bash', but it can be almost anything. If the program requires X, you may have to add a wrapper of some sort to establish the correct $DISPLAY setting; possibly more.
Last edited by theNbomr; 01-25-2008 at 11:38 AM.