See this bug:http://jira.jboss.com/jira/browse/JBIDE-190
I used the eclipse updater to grab 15m1 for my copy of eclipse 3.1 and I got the same error that you do.
I then downloaded the 15m2 and installed that over, still the same error.
I am now giving up and am downloading the package containing the JbossIDE and eclipse.
I found a working configuration.
I started with Eclipse 3.0 and I installed jboss 4.0.2 and JBossIDE-1.4.1-jre1.5-e31 ( I have 1.5 jdk) . This is absolutely working !
Now I'll try to update this configuration via software updating section into Eclipse.