Since today, the alpha UseBB 2 source code has been added into our public Subversion repository.
The website has not been updated yet, but you can find SVN connection information over at https://sourceforge.net/svn/?group_id=93103. The web interface can be found at http://usebb.svn.sourceforge.net/viewvc/usebb/.
Note all code changes are also sent to the checkins mailing list. For subscription information, see https://lists.sourceforge.net/lists/listinfo/usebb-checkins.
What's next is to compose coding guidelines for anyone wishing to contribute code. As usual, all code is kept under the GNU General Public License.