I have wanted core SQL Server support for some time now. But, like many of you, I have either been too busy with getting my own site running on it (in my case SQL Server 2000) or I have had difficulties finding enough concentrated support to get things rolling. To my knowledge there is one semi-active discussion on mainly 2005 support here: http://drupal.org/node/74308 and another that has some use for historical purposes here: http://drupal.org/node/132565
I admit that I have focused on 2000 due to restrictions beyond my control, so I am not up-to-speed with the latest developments for 2005 support, but please consider this the place to post that information. Let's see what we can do to formalize all of this a bit more. I know that Dries has interest in seeing this succeed.