Quite many people installed it on Portal 2.6 GA, download counter says that almost 3K + people using svn.
What AS do you use?
Make sure you downloaded latest binary from Forums downloads page and a version compatible with Portal 2.6.
Did you modify any of JSF or Tomahawk libs or does any other app in your deploy dir contains those libs?
JBoss Forums Lead
JBoss Labs Team
JBoss Forums requires myFaces.
The easiest way is to install JBoss AS 4.0.5 as of now you can use the bundle version if you want.
JBoss Forums should embed the myFaces libraries ;)