Migrating your website/CMS to Drupal
I've noticed recently that there is quite a significant number of Drupal projects that involve migrating an existing site form platform X or CMS Y to Drupal, including behemoths like economist.com and examiner.com. We have had requests from our own clients too.
Migration is always a tricky business, but the good thing about migrating to Drupal is that a lot of people has been there and done that, and there is information online to help you through that, whether you're migrating from ColdFusion, Joomla, Wordpress or your own custom system.
There is even a Drupal module to help you with the migration, called, well, Migrate (does what it says on the tin).
Or have a look at a Drupal migration handbook covering various source platforms.