Is there a SQL Server Provider for Drupal? I know that there is a SQL Server Client for PHP in beta and I wonder whether it would be hard to change the database provider to point to SQL Server instead of MySQL.
Drupal core only officially supports MySQL and PostgreSQL as database providers for Drupal 6, with work being done to support SQLite for Drupal 7.
The best resource I could find was this item (http://drupal.org/node/74308) in the issue queue, but it hasn't had activity in a few months. There are a few patches posted with database layer files, but you will have to thoroughly test them for your use.
There were a couple postings on groups.drupal.org about using SQL Server for sites as well that may be worth checking out: http://groups.drupal.org/taxonomy/term/844.
Drupal 6 does not support SQL Server. Drupal 7's new database layer will be based on PDO. While PDO doesn't yet work directly with SQL Server, both PDO and SQL Server work with ODBC. So, in theory, you'll be able to use SQL Server with Drupal 7. How that works in practice remains to be seen.