what did you do to activate JSF 1.2? JBoss 4.2 bundles MyFaces 1.1, and if your own app contains a custom JSF implementation, this should result in problems.
I assume that changed the config to bundle your own JSF implemetation (otherwise, youh would have been hit bit different errors earlier).
I changed the config to bundle Icefaces. My application have 50 or 60 forms and only fails in one of them. I don't think that the problem is the configuration.