I just added this myself, and since there is a mod only for SMF 1.x I decided to share with all.
I'm unsure if this will work on all software versions, well here it is.
// Show the "Powered by" and "Valid" logos, as well as the copyright. Remember, the copyright must be somewhere!
//Google PageRank CODE START
echo '
</div>';
echo'<br />
<div align="center">
<a href="http://www.free-pagerank-checker.com/" title="Free PageRank checking tool" target="_blank">
<script type="text/javascript">
document.write("<img src="http://www.free-pagerank-checker.com/pr.php?url="+document.URL+"" border="0" alt="PageRank" />");
</script>
</a>
</div>
<br />';
//Google PageRank CODE END
The code is courtesy of www.free-pagerank-checker.com and I take no credit for the code, but just showing how to integrate it with SMF, the icon will be placed right above the SMF version information right at the bottom.
the codes can be obtained here (http://www.free-pagerank-checker.com/pagerank_button_code.php)
Pretty Nice, thanks for the help.