If the terminal window has anything more than a single line ending with "$" or "#," it may be that a script is being run by terminal at startup/login. If so, preventing terminal from starting will also prevent that script from running.
In addition to the account login items, it could be started by the script /etc/rc.local in 10.5, or several other /etc/rc* scripts in 10.4 and earlier.
There could also be a script or plist file in one of the following directories:
/Library/StartupItems
/System/Library/StartupItems
/--your user directory--/Library/LaunchAgents
/Library/LaunchAgents
/Library/LaunchDaemons
/System/Library/LaunchAgents
/System/Library/LaunchDaemons