Curious to know if it is possible, and simple to create a single sign on between Drupal 6 and Sharepoint 2007?

We have a large members area that is developed in Sharepoint that we would like to keep, but are considering changing the non-logged in user site to Drupal. The question is how do they integrate, and even do they need to integrate?

The key part is sharing the navigation from Drupal to Sharepoint so the experience is similar, and that a member would have access to their member navigation from the Drupal site.

Is integrating the two systems like this a really bad idea? Is it a simple solution? Does anyone have any experience?

Looking forward to updates!

Thanks as always,

Chris