Firefox + Firebug, right click, inspect element shows that the t_outer class has :
margin-bottom:5px;
but you can't just edit that to 0, because you'd affect all the other wrappers that 'depend' on that.
you could edit footer.tpl and not do {$tbopen}
and instead put in:
<table width="100%" class="t_outer" style="margin-bottom:0;" cellpadding="0" cellspacing="0">
<tr>
<td>
<table width="100%" class="t_inner" cellpadding="0" cellspacing="1">
allowing the inline style to kill the bottom margin for JUST that one case