When trying a fresh install of JBossIDE (by downloading the jre 1.5 ALL version from sf.net and installing into eclipse 3.1M6) I get the following stack trace:

java.lang.NoClassDefFoundError: org/eclipse/jdt/internal/ui/text/java/ResultCollector
at org.jboss.ide.eclipse.jdt.j2ee.jsp.ui.editors.JSPConfiguration.getContentAssistant(JSPConfiguration.java:192)
at org.eclipse.jface.text.source.SourceViewer.configure(SourceViewer.java:332)
at org.eclipse.ui.texteditor.AbstractTextEditor.createPartControl(AbstractTextEditor.java:2502)
at org.eclipse.ui.texteditor.StatusTextEditor.createPartControl(StatusTextEditor.java:53)
at org.eclipse.ui.texteditor.AbstractDecoratedTextEditor.createPartControl(AbstractDecoratedTextEditor.java:303)
at org.eclipse.ui.internal.EditorManager.busyRestoreEditorHelper(EditorManager.java:1237)
at org.eclipse.ui.internal.EditorManager.busyRestoreEditor(EditorManager.java:1102)
at org.eclipse.ui.internal.EditorManager$7.run(EditorManager.java:1064)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:69)
at org.eclipse.ui.internal.EditorManager.restoreEditor(EditorManager.java:1062)
at org.eclipse.ui.internal.EditorManager$Editor.getEditor(EditorManager.java:1648)
at org.eclipse.ui.internal.EditorManager$Editor.getPart(EditorManager.java:1639)
at org.eclipse.ui.internal.PartPane.requestActivation(PartPane.java:235)
at org.eclipse.ui.internal.EditorPane.requestActivation(EditorPane.java:88)
at org.eclipse.ui.internal.PartPane.setFocus(PartPane.java:270)
at org.eclipse.ui.internal.EditorPane.setFocus(EditorPane.java:116)
at org.eclipse.ui.internal.presentations.PresentablePart.setFocus(PresentablePart.java:134)
at org.eclipse.ui.internal.presentations.newapi.TabbedStackPresentation$1.handleEvent(TabbedStackPresentation.java:92)
at org.eclipse.ui.internal.presentations.newapi.AbstractTabFolder.fireEvent(AbstractTabFolder.java:263)
at org.eclipse.ui.internal.presentations.newapi.AbstractTabFolder.fireEvent(AbstractTabFolder.java:268)
at org.eclipse.ui.internal.presentations.newapi.AbstractTabFolder.handleMouseDown(AbstractTabFolder.java:322)
at org.eclipse.ui.internal.presentations.newapi.AbstractTabFolder$3.mouseDown(AbstractTabFolder.java:79)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:132)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:82)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:842)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:2894)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:2527)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:1570)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:1534)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:306)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:143)
at org.eclipse.ui.internal.ide.IDEApplication.run(IDEApplication.java:103)
at org.eclipse.core.internal.runtime.PlatformActivator$1.run(PlatformActivator.java:228)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:156)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.core.launcher.Main.invokeFramework(Main.java:315)
at org.eclipse.core.launcher.Main.basicRun(Main.java:268)
at org.eclipse.core.launcher.Main.run(Main.java:942)
at org.eclipse.core.launcher.Main.main(Main.java:926)

Verified that jdt.jar does, in fact, NOT have that class file:

0 Fri Apr 01 17:05:52 CST 2005 org/eclipse/jdt/ui/text/java/
15514 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/CompletionProposalCollector.class
1680 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/CompletionProposalComparator.class
8096 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/CompletionProposalLabelProvider.class
441 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IInvocationContext.class
249 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IJavaCompletionProposal.class
561 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IJavadocCompletionProcessor.class
421 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IProblemLocation.class
483 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IQuickAssistProcessor.class
466 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/IQuickFixProcessor.class
1820 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/JavaTextMessages.class
644 Fri Apr 01 17:05:52 CST 2005 org/eclipse/jdt/ui/text/java/JavaTextMessages.properties
0 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/hover/
251 Fri Apr 01 17:05:46 CST 2005 org/eclipse/jdt/ui/text/java/hover/IJavaEditorTextHover.class