Step one is to create a new admin user account, log into it, and see if the problem persists.
Step two is to repost your issue and results from step one to the Unix forum.27" i7 iMac, 12 GB RAM, Mac OS X (10.6.7), G4 450 MP 1.5 GB RAM w/(10.5.8/10.4.11/9.2.2)
Either you have a problem with the shell value in Directory Services or something is amiss in the file /etc/shells.
From System Preferences > Accounts (unlock if necessary) > Select your account holding down the control key > Select Advanced Options. Change the Login Shell: value to /bin/bash then click the OK button. Exit System Preferences. Log Out or restart your computer. Hopefully this works.