Created diagram, setup two participants, dragged start event, dragged choreography task, assigned participants, created sequence link from start event to choreo task, added another choreo task, selected all three nodes and selected the alignment->centre option on the context menu.

Firstly the centre option is vertical - was not clear whether vertical or horizontal.

The error log then had two errors which appear to be the same:

eclipse.buildId=I20100608-0911
java.version=1.6.0_18
java.vendor=Sun Microsystems Inc.
BootLoader constants: OS=linux, ARCH=x86, WS=gtk, NL=en_US
Framework arguments: -product org.eclipse.epp.package.jee.product
Command-line arguments: -os linux -ws gtk -arch x86 -product org.eclipse.epp.package.jee.product

Error
Wed Mar 09 12:17:28 GMT 2011

java.lang.NullPointerException
at org.jboss.bpmn2.editor.core.utils.AnchorUtil.updateEdge(AnchorUtil.java:169)
at org.jboss.bpmn2.editor.core.utils.AnchorUtil.reConnect(AnchorUtil.java:154)
at org.jboss.bpmn2.editor.core.features.DefaultBpmnMoveFeature.postMoveShape(DefaultBpmnMoveFeature.java:35)
at org.jboss.bpmn2.editor.core.features.MoveFlowNodeFeature.postMoveShape(MoveFlowNodeFeature.java:81)
at org.eclipse.graphiti.features.impl.DefaultMoveShapeFeature.moveShape(DefaultMoveShapeFeature.java:65)
at org.eclipse.graphiti.internal.command.MoveShapeFeatureCommandWithContext.execute(MoveShapeFeatureCommandWithContext.java:81)
at org.eclipse.graphiti.internal.command.CommandContainer.execute(CommandContainer.java:81)
at org.eclipse.graphiti.ui.internal.command.GefCommandWrapper.execute(GefCommandWrapper.java:51)
at org.eclipse.gef.commands.CompoundCommand.execute(CompoundCommand.java:111)
at org.eclipse.graphiti.ui.internal.editor.EmfOnGefCommand.execute(EmfOnGefCommand.java:58)
at org.eclipse.graphiti.internal.command.GFPreparableCommand2.doExecute(GFPreparableCommand2.java:37)
at org.eclipse.emf.transaction.RecordingCommand.execute(RecordingCommand.java:135)
at org.eclipse.emf.workspace.EMFCommandOperation.doExecute(EMFCommandOperation.java:119)
at org.eclipse.emf.workspace.AbstractEMFOperation.execute(AbstractEMFOperation.java:150)
at org.eclipse.core.commands.operations.DefaultOperationHistory.execute(DefaultOperationHistory.java:511)
at org.eclipse.emf.workspace.impl.WorkspaceCommandStackImpl.doExecute(WorkspaceCommandStackImpl.java:208)
at org.eclipse.emf.transaction.impl.AbstractTransactionalCommandStack.execute(AbstractTransactionalCommandStack.java:165)
at org.eclipse.graphiti.ui.internal.editor.GFWorkspaceCommandStackImpl.execute(GFWorkspaceCommandStackImpl.java:47)
at org.eclipse.emf.transaction.impl.AbstractTransactionalCommandStack.execute(AbstractTransactionalCommandStack.java:219)
at org.eclipse.graphiti.ui.internal.editor.GFWorkspaceCommandStackImpl.execute(GFWorkspaceCommandStackImpl.java:39)
at org.eclipse.graphiti.ui.internal.editor.GFCommandStack.execute(GFCommandStack.java:108)
at org.eclipse.gef.ui.actions.WorkbenchPartAction.execute(WorkbenchPartAction.java:83)
at org.eclipse.gef.ui.actions.AlignmentAction.run(AlignmentAction.java:253)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:498)
at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:584)
at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:501)
at org.eclipse.jface.action.ActionContributionItem$5.handleEvent(ActionContributionItem.java:411)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1258)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3552)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3171)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2629)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2593)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2427)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:670)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:663)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:115)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:369)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:619)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:574)
at org.eclipse.equinox.launcher.Main.run(Main.java:1407)

When I tried to undo the action, it did not do anything.