I see a lot of these in the workspace log file:

!ENTRY org.eclipse.core.resources 4 2 2008-09-08 18:25:57.937
!MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
!STACK 0
java.lang.NullPointerException
at org.jboss.ide.eclipse.archives.core.WorkspaceChangeListener$2.visit(WorkspaceChangeListener.java:72)
at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:68)
at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:48)
at org.jboss.ide.eclipse.archives.core.WorkspaceChangeListener.resourceChanged(WorkspaceChangeListener.java:68)
at org.eclipse.core.internal.events.NotificationManager$2.run(NotificationManager.java:288)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:37)
at org.eclipse.core.internal.events.NotificationManager.notify(NotificationManager.java:282)
at org.eclipse.core.internal.events.NotificationManager.broadcastChanges(NotificationManager.java:148)
at org.eclipse.core.internal.resources.Workspace.broadcastPostChange(Workspace.java:313)
at org.eclipse.core.internal.resources.Workspace.endOperation(Workspace.java:1022)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:1809)
at org.eclipse.wst.validation.internal.operations.ValidationOperation$1.done(ValidationOperation.java:1398)
at org.eclipse.core.internal.jobs.JobListeners$3.notify(JobListeners.java:39)
at org.eclipse.core.internal.jobs.JobListeners.doNotify(JobListeners.java:112)
at org.eclipse.core.internal.jobs.JobListeners.done(JobListeners.java:152)
at org.eclipse.core.internal.jobs.JobManager.endJob(JobManager.java:558)
at org.eclipse.core.internal.jobs.WorkerPool.endJob(WorkerPool.java:105)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:72)

These exceptions does not prevent the normal work of the ide.