For applications that require a login, it would be very nice to be able to specify the valid values for login forms. As an example:
I have an app with public and authenticated parts.
I would want tarantula to run through the public part jamming anything into fields and clicking links.
Now I have to login to access my private section.
I would want tarantula to try jamming info into the login form, and then use the right username and password to get through the login form and start testing the private sections.