http://wordpress.visitmix.com

This patch makes wordpress workable with MS Sql Server 2005, 2008 and Azure Databases

You may either download the entire wordpress + package or you may download the standalone 
patch and overwrite the files in your wordpress directory. If you do the later to an 
existing wordpress installation it is strongly recommended that you backup your database, 
config, the files to be overwritten, and also make a WP export of your blog (which you 
would want to import once you setup wordpress for your MS database).

These patches uses the mssql PHP drivers for Sql Server 2005, 2008 and you may also use 
the sqlsrv drivers to connect to a SQL Azure DB.

http://www.microsoft.com/sqlserver/2005/en/us/PHP-Driver.aspx