Ok, I found it.
I only found this information on Shigeru Chiba's personal page. It is a pity that the JBoss.org and JBoss.com sites only present the projects and do not explain anywhere on the site how to fetch the sources for these projects.
Such an attitude is really bad for the open-source world.
Here's how you can access the JBoss CVS. This was taken from Mr. Chiba's personal page, at http://www.csg.is.titech.ac.jp/~chiba/javassist/ .
The Latest Snapshot
The latest source code is available from CVS. You can check all the source files out from the CVS repository with an anonymous account (empty password) with the following two commands:
cvs -d :pserver:email@example.com:/cvsroot/jboss login
cvs -z3 -d :pserver:firstname.lastname@example.org:/cvsroot/jboss co javassist
Note that the login password is empty. Hit enter at the password prompt.
You can find other projects there as well.
If you came here from a search engine, keep in mind that this information was gathered in July 2007 from the personal page of Mr. Chiba and may not be accurate in the future. I really hope that JBoss.org will publish the CVS information along with the Javassist page at JBoss.org so it is easier to find.