One way is to use custom plugin. In fact, such custom plugin doesn't need to do anything, only declare new toolbar contribution using existing undo/redo commands.
I've built such plugin for you: http://www.foglyn.com/misc/undoredo_1.0.0.jar. There is absolutely no code, only plugin.xml:
<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.4"?>
<plugin>
   <extension
         point="org.eclipse.ui.menus">
      <menuContribution
          locationURI="toolbar:org.eclipse.ui.main.toolbar?after=additions">
         <toolbar
               id="undoredo.toolbar">
            <command
                  commandId="org.eclipse.ui.edit.undo"
                  id="undoredo.undo"
                  style="push">
            </command>
            <command
                  commandId="org.eclipse.ui.edit.redo"
                  id="undoredo.redo"
                  style="push">
            </command>
         </toolbar>
      </menuContribution>
   </extension>
</plugin>
And MANIFEST.MF:
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Undoredo
Bundle-SymbolicName: undoredo;singleton:=true
Bundle-Version: 1.0.0
Bundle-RequiredExecutionEnvironment: J2SE-1.5
Require-Bundle: org.eclipse.ui
You can download it, and drop into your 'dropins' directory of Eclipse, restart, and you'll see Undo/Redo buttons on your toolbar.
Works for me in Eclipse 3.4 and Eclipse 3.5M7.