Using BBEdit's Search function, I made a multi-file search through all of the iWeb project's .html files, searching for...
</body>
...and telling BBedit to replace it with this:
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "##-######-#";
urchinTracker();
</script>
</body>
Replace the ##-######-# with your site's tracking ID. Now just save this as a search pattern, and every time after publishing your site to a folder (~/Sites in my case), just apply it to that folder.
Using AppleScript, it should even be possible to make this a more or less automatic task. It's not very sophisticated, but for my needs it does the job well. It even works if you are publishing to your .Mac account -- simply apply the rules on your specific iDisk's iWeb Folders. (For reference, this hint comes from this entry on my blog.)

