Work is currently underway to provide an extension so that SilverStripe can run on Microsoft SQL Server 2008. The SilverStripe module allowing for this connection to the SQL Server is currently still being developed, but the alpha version according to Sigurd Magnusson does work.
Other than that, we're using the code with success internally, and know others in the community are too. We're mentioning this SQL Server milestone in the blog to let you know that the feature is well on its way, and to allow careful developers a chance to try it out, provide feedback, and files bugs before we mark it as a publicly stable module.
SilverStripe, as with many PHP-based content management systems, has historically only supported MySQL for it's database. Besides MS SQL, SilverStripe also hopes to be able to run on additional databases such as PostgreSQL and SQLite.