[Lynne]

check in the showthread_quickreply template (and similar in postbit_quickedit and editor_toolbar_on) and see if these lines have been modified:

     <if condition="$show['wysiwyg_compatible']">
        <td><img src="$stylevar[imgdir_editor]/separator.gif" width="6" height="20" alt="" /></td>
        <td><div class="imagebutton" id="{$editorid}_cmd_switchmode"><img src="$stylevar[imgdir_editor]/switchmode.gif" width="21" height="20" alt="$vbphrase[switch_editor_mode]" /></div></td>
    </if>

edit: have you tried disabling your plugins to see if this works without them?