I guess, it's because of the changes to the location of the libraries that have been moved to a common/lib folder. Maybe you can try what this guy did http://www.coderanch.com/t/424328/JBoss/JBoss-GA-Eclipse
according to the eclipse bugreport at https://bugs.eclipse.org/bugs/show_bug.cgi?id=257823, this was included in the WTP 3.0.4 branch yeasterday.
So you might try to build an eclipse 3.4 with a WTP 3.0.4 comitters build from http://build.eclipse.org/webtools/committers/
Thank you for both replies.
I followed Wolfgang's proposal, and copied the file https://bugs.eclipse.org/bugs/attachment.cgi?id=120132 into eclipse/plugins/org.eclipse.jst.server.generic.jboss_1.5.205.v200805140145/servers/jboss5.serverdef.
That solved the case.
I'm using JBoss Tools which is based on the WTP plugin. WTP provides a generic JBossAS adapter - JBossTools is dedicated to JBoss and provides a lot of good and stable features.