I'll take a look at this tonight, if I have time. If it helps, here's the edit form with all non-form tags removed. It might show you something you're missing.
<form id="editform" name="editform" method="post" action="/w/index.php?title=Wikipedia:Sandbox&action=submit" enctype="multipart/form-data">
<input type='hidden' value="" name="wpSection" />
<input type='hidden' value="20060103120224" name="wpStarttime" />
<input type='hidden' value="20060103120149" name="wpEdittime" />
<input type='hidden' value="" name="wpScrolltop" id="wpScrolltop" />
<textarea tabindex='1' accesskey="," name="wpTextbox1" id="wpTextbox1" rows='25' cols='80' >[page text]</textarea>
<input tabindex='2' type='text' value="" name='wpSummary' id='wpSummary' maxlength='200' size='60' />
<input tabindex='5' id='wpSave' type='submit' value="Save page" name="wpSave" accesskey="s" title="Save your changes [alt-s]"/>
<input tabindex='6' id='wpPreview' type='submit' value="Show preview" name="wpPreview" accesskey="p" title="Preview your changes, please use this before saving! [alt-p]"/>
<input tabindex='7' id='wpDiff' type='submit' value="Show changes" name="wpDiff" accesskey="v" title="Show which changes you made to the text. [alt-d]"/>
</form>