I have a similar idea for our support-website. Of course customers and partners have their members stored as umbraco members, but it could be cool if our internal people were taken from Active Directory. This way they know their credentials and passwords. And it is always synchronized with the website.

Since this is not the highest priority, I have only taken a short look (5 minutes or so) at this issue. It should be possible to create a login user control that checks against Microsoft's Directory Services (System.DirectoryServices namespace).
Again, this is only in theory. We all know Microsoft doesn't always make things easy ;)

More information on the namespace can be found here:
http://msdn2.microsoft.com/en-us/library/system.directoryservices.aspx