I'm having this problem as well. Error log shows:
Java Model Exception: Java Model Status [lah does not exist]
Downgrading to 3.1M4 doesn't help. JBoss-IDE 1.5M1 doesn't seem to work with eclipse 3.1M4 either.
So basically, I can't get JBoss-IDE 1.5M1 to work with any eclipse 3.1 version on Windows XP :
eclipse 3.1M4 - doesn't support JSP editor
eclipse 3.1M5a - can't create new J2EE 1.4 projects
eclipse 3.1M6 - doesn't support JSP editor
Interestingly enough, I don't have any problems with Eclipse 3.1M5a and JBoss-IDE 1.5M1 on Solaris/GTK.
I guess I'm going to have to downgrade to JBoss-IDE 1.4 and eclipse 3.1M4. Any help is appreciated.