Redmine

This is infact already available. You have to do some wiki-searching, but the info is there. Here is the start:

http://www.redmine.org/wiki/redmine/RedmineSettings#28

http://www.redmine.org/wiki/redmine/HowTo_to_handle_SVN_repositories_creation_and_access_control_with_Redmine

Good luck!

The easiest way is described here: Automating_repository_creation
IMHO, it's not a good idea to have the repositories created through the web interface.