I'll have look and see if I can find the Orion stuff - No offence, but it could save one wasting their time on this, when they could be doing other things.
 
Edit - It's AJAX:
Found it! Don't know if it will help - - But!
<td class="footer" colspan="2" align="center" height="28">{S_HIDDEN_FORM_FIELDS}
<input type="submit" tabindex="5" name="preview" onclick="return AJAXPreview({S_IS_PM}, {S_EDIT_POST_ID});" class="mainoption" value="{L_PREVIEW}" />
  <input type="submit" accesskey="s" tabindex="6" name="post" class="mainoption" value="{L_SUBMIT}" onClick="this.value='{OF_WARTEN}'" />
 
Of Warten = Please Wait.  
I also know that myBB had the same problem with IE, and they resorted to using js to submit the form. Only difference is - They used a "Spinner" like I posted earlier. 
