With the recent release of Drupal 7, its PDO database abstraction layer means that Drupal 7 can technically support a variety of databases (provided they have the proper Drupal 7 database interface).
Of course, what Drupal 7 + PDO means is that yet another barrier to entry for "Microsoft Shops" to use Drupal has faded away.  One of the biggest reasons.... ahem.. excuse to not use Drupal (or just about any other Open Source CMS for that matter) was that the CMS needed to run on _______  (fill in the blank here with whichever database you like that is NOT SQL Server).  Not to mention the poor support that used to plague PHP on IIS.
Thanks to hard work by Commerce Guys, the sqlsrv project was born, giving Drupal 7 the required SQL Server database driver. That being said, you may notice that the sqlsrv project is still in an Alpha release state at the time of writing of this blog article (Update -- January 31st 2011  -- sqlsrv is now in RC status!). The Alpha state of modules generally turns people away from using it. However I strongly urge people to continue to use it and help work out any bugs with sqlsrv to bring it to a stable release.
Regardless of the release state of sqlsrv and some of the outstanding issues in its issue queue, it does run quite well and actually didn’t give me any major issues when working on a Maestro project for a client.