It is definitely possible to run WordPress connecting to MS SQL Server. We are running such an instance in production at my company. Here are two posts that describe the process of using WordPress with MS SQL Server and IIS. I am using such a configuration right now on my local box.
http://techsugar.wordpress.com/2011/03/02/how-to-install-wordpress-on-iis-7-5-using-sql-server-2008/
http://wordpress.visitmix.com/development/installing-wordpress-on-sql-server