Starting up Eclipse to do a PDE build as part of the uberbuilder with Eclipse 3.6 & latest basebuilder (first attempt), we get this:

!SESSION 2010-05-26 17:05:20.741 -----------------------------------------------
eclipse.buildId=200905221710
java.version=1.6.0_18
java.vendor=Sun Microsystems Inc.
BootLoader constants: OS=linux, ARCH=x86_64, WS=gtk, NL=en_US
Framework arguments: -application org.eclipse.ant.core.antRunner -f /qa/hudson_ws/workspace/jbosstools-continuous-3.2.helios/org.jboss.ide.eclipse.releng/builders/product/productBuild.xml run.builders
Command-line arguments: -application org.eclipse.ant.core.antRunner -data /qa/hudson_ws/workspace/jbosstools-continuous-3.2.helios/jbds-build/workspace -f /qa/hudson_ws/workspace/jbosstools-continuous-3.2.helios/org.jboss.ide.eclipse.releng/builders/product/productBuild.xml run.builders

!ENTRY org.eclipse.equinox.ds 4 0 2010-05-26 17:05:26.014
!MESSAGE
!STACK 0
org.osgi.framework.BundleException: The bundle "org.eclipse.equinox.ds_1.2.0.v20100507 [2]" could not be resolved. Reason: Missing Constraint: Import-Package: org.eclipse.equinox.internal.util.event; version="1.0.0"
at org.eclipse.osgi.framework.internal.core.AbstractBundle.getResolverError(AbstractBundle.java:1317)
at org.eclipse.osgi.framework.internal.core.AbstractBundle.getResolutionFailureException(AbstractBundle.java:1301)
at org.eclipse.osgi.framework.internal.core.BundleHost.startWorker(BundleHost.java:319)
at org.eclipse.osgi.framework.internal.core.AbstractBundle.resume(AbstractBundle.java:374)
at org.eclipse.osgi.framework.internal.core.Framework.resumeBundle(Framework.java:1067)
at org.eclipse.osgi.framework.internal.core.StartLevelManager.resumeBundles(StartLevelManager.java:561)
at org.eclipse.osgi.framework.internal.core.StartLevelManager.resumeBundles(StartLevelManager.java:546)
at org.eclipse.osgi.framework.internal.core.StartLevelManager.incFWSL(StartLevelManager.java:459)
at org.eclipse.osgi.framework.internal.core.StartLevelManager.doSetStartLevel(StartLevelManager.java:243)
at org.eclipse.osgi.framework.internal.core.StartLevelManager.dispatchEvent(StartLevelManager.java:440)
at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:227)
at org.eclipse.osgi.framework.eventmgr.EventManager$EventThread.run(EventManager.java:337)

!ENTRY org.eclipse.update.configurator 4 0 2010-05-26 17:05:26.605
!MESSAGE Could not install bundle plugins/org.eclipse.equinox.launcher_1.1.0.v20100507.jar Bundle "org.eclipse.equinox.launcher" version "1.1.0.v20100507" has already been installed from: update@plugins/org.eclipse.equinox.launcher.jar

!ENTRY org.eclipse.osgi 4 0 2010-05-26 17:05:26.648
!MESSAGE An unexpected runtime error has occurred.
!STACK 0
javax.xml.parsers.FactoryConfigurationError: Provider org.apache.xerces.jaxp.SAXParserFactoryImpl not found
at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:134)
at org.eclipse.core.runtime.internal.adaptor.EclipseAdaptorHook$ParsingService.createService(EclipseAdaptorHook.java:147)
at org.eclipse.core.runtime.internal.adaptor.EclipseAdaptorHook$ParsingService.getService(EclipseAdaptorHook.java:139)

http://hudson.qa.jboss.com/hudson/view/DevStudio/job/jbosstools-continuous-3.2.helios/ws/jbds-build/org.eclipse.releng.basebuilder/configuration/1274907921117.log

No idea what's causing this, much less how to fix.