Well, you can make another table within the post itself. And have the cell with the post be 100% tall, and then the signature in a seperate cell... That should work.
Like this:
Code:
<TABLE height="100%" border="0">
<TR>
<TD height=100%><FONT
face="verdana, arial, helvetica" size=2>
<p>$post[message]</p>
<p>$post[attachment]</p>
</TD>
</TR>
<TR>
<TD>
<FONT
face="verdana, arial, helvetica" size=2>
<p>$post[editedby]</p>
<p>$post[signature]</p></font>
</TD>
</TR>
</TABLE>
Hope that helps.