Create a new web project (I did a Seam web app) in JBDS 4.0.0.GA. Open the "Project archives" view. Attempt to build a WAR archive. You will get an error message "Could not initialize class de.schlichtherle.io.ArchiveControllers".
Workaround (sort of, I still have problems creating a WAR archive, but it fixes this specific message) is to download truezip-6.jar and place in the studio/plugins folder.