I created this Python script to automatically convert a site in iWeb into a template, and install it in the iWeb application. The following description is taken from the comments at the top of the script:
This script will convert an iWeb site into an iWeb template and install it. The usage is fairly simple. Just create a site in iWeb with the name that you want the template to have. The site should should have each of the standard pages (i.e. About Me, Welcome, Photos, etc.). Once you have a site that looks the way you want, open the Terminal application and run:The only thing you have to do manually is create the thumbnails. Read the comments at the top of the script for more information.where site-name is the name of the site you want to convert to a template.site2template site-name
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060225091102170