I recently built a migration script as a proof of concept that my college could make the change to Drupal pretty seamlessly.
Our current CMS (reddot) has the ability to output valid XHTML files. I built a script in classic ASP script (am currently porting it to PHP) that scraped these XHTML files using Xpath and output them into a CSV file. I just made sure to consistently name elements (e.g. - <div id="header">) so that Xpath could find them.