MSSql or SQL Server Wrapper for custom module?
Since you didn't specify a Drupal version, I'll try to answer for both Drupal 6 and 7.

Drupal 7

See http://drupal.org/project/sqlsrv

Drupal 6

Even if there would be SQL Server integration in the official db-api, it wouldn't help you much because with Drupal 6, you can only connect to multiple databases of the same type. This is why https://github.com/dereine/views_pgsql was created for example.

However, it might be an option to use the DBTNG project, which is a backport of the new Drupal 7 database abstraction layer and use that in combination with the sqlsrv project. I haven't tested that but it could work. And if not, maybe with a few changes (which you could contribute back)