Migration data from Tridion to Drupal
There are a number of api/integration points for SDL Tridion, depending on your version you have:

SDL Tridion 2009 and previous -> TOM (Tridion Object Model) a COM+ API you can use in a .NET project.

SDL Tridion 2011 -> TOM (as before) or the SDL Tridion Core Service (.NET web service)

I'd recommend using these APIs to extract the information you need from Tridion into XML data in the structure that makes it easy for you to work with.

You can then use something like the services module http://drupal.org/project/services to process and import your XML.

Also maybe of use... SDL Tridion is a component based CMS and content is based on a specific Schema items.   A schema is likely to translate to a drupal content type pretty well, but if you're trying to achieve the sort of flexible layout that you can get from SDL Tridion (page/component templates) you'll likely need to be using the Panels module.. or something similar.

Let me know if you need some more information.