I had been looking at doing the same thing recently, but decided that I would be better off setting up MySQL just to serve Mediawiki. Consider the advantages of each:
- Is a supported database, so you'll get better answers if something goes wrong.
- Will definitely be supported by patches and upgrades.
- Is very well tested through Wikipedia. You're less likely to see "strange" behaviour.
- Easy setup and widely available documentation - whereas you will probably need to carefully document the tricks you use to get MediaWiki working with MSSQL.
- Can leverage existing backup methods in a homogeneous environment
- Can assign permissions based on Active Directory
It took me less than an hour to configure a virtual machine with a LAMP stack, then install and configure MediaWiki. Backup is a crontab that dumps the MySQL database onto the fileserver. Your mileage may vary, but you'll probably end up saving a lot of time by going with the supported solution.