any modules could autoly create a new subsite? or script is ok

Multisite manager or Course manager (poor name but it does exactly these things)

Here's the links to them, I haven't used Multisite in awhile but it worked pretty well back in the day. Course manager is a project I manage and it's current;y just in cvs so I'd recommend trying multisite manager.

http://drupal.org/project/multisite_manager (I haven't used the 6 version but I'll assume it works the same)
http://drupal.org/project/course_manager

"Plaguing the world with Drupal; One Plone, Moodle, Wordpress, Joomla user at a time since 2005." ~ btopro

http://elearning.psu.edu/
http://elearning.psu.edu/projects/
http://elearning.psu.edu/drupalineducation/

I have a script that I use. I have a site that I use as my template that only gets changed when I want to change my default template. The script is a little specific, but could easily be adapted. In order:

1. Copies the depttemplate settings folder to the new folder
2. Changes the database name in settings
3. It creates a new database
4. Exports the template db
5. Imports the template db into the new database
6. Updates the new sites site name changing it from the default templates site name.
7. Updates the site email address and other things that are specific to my install.
8. Adds a list of users that I specified and puts them in the proper role. I currently set a random password for these users because I use cas authentication and the passwords aren't used.
9. Lastly it adds the sites to a sites.ini file that I use. This file is used in conjunction with a back-ported d7 feature that allows me to do things as point multiple domains to the same sites folder.

Attached is a cleaned up version of my script. It does need adjustment before it will work for your site. Especially adding users.

There are two files. You can run bash site_install.sh to see the command line options. All are required at the moment unless you adjust the script.

Hope this helps.