I was interested in getting Drupal to run with MSSQL rather than MySQL so I set out to see if I might be able to hack the code to support MSSQL whether through PHP MSSQL driver or through Microsoft SQLSRV driver...
I spent several weeks on this effort and here are some quick write up on that effort.
There was some effort prior to Drupal 6 and some were able to get it running with MSSQL at some point but the effort was completely stalled indefinitely when Drupal 6 was released.
The reason that I found was prior to Drupal 6 the SQL statement was in a file so to create support for MSSQL were just a matter of creating the SQL files that contain MSSQL SQL statement but in Drupal 6 those SQL files are no longer used instead they were dynamically built at run time...
So to create any kind of support for MSSQL in Drupal 6 is an IMPOSSIBLE task!
But with Drupal 7 there is some effort that is working to get MSSQL support with the help from Microsoft and some heavyweight Drupal developers.
Stay tuned to see how that effort turns out with Drupal 7 for MSSQL support.
Note: The MSSQL support in Drupal 7 will likely be for Microsoft version of the driver which is "SQLSRV" not PHP version of the driver "MSSQL"