I've uploaded my code for a MSSQL-compatible Drupal 5 installation. The only folder that didn't commit properly was modules/color, so I'll try to get that working soon - hopefullly that doesn't prevent you from installing and using a non-color module theme. You can find the files on cvs.drupal.org at /contributions/sandbox/pcorbett/drupal-mssql. You'll need to checkout all the folders and then move the contents of the root_folder_files into your root folder (CVS doesn't let me commit files that aren't contained in a folder).
If you need help getting started, let me know. This may or may not be helpful to anyone - I'm using this in a production environment, but with some 3rd party apps such as FreeTDS to talk between PHP and SQL Server using phpdblib.dll instead of the typical php_mssql.dll (which poses some restrictions on DB calls made from PHP and making it difficult to use Drupal).
There are some php.ini settings that may not be obvious, so if you have issues, feel free to contact me.
Hopefully we'll have D6 ready for SQL Server shortly. I've heard rumors that others are already working on this and there are some new developments that should make using FreeTDS unnecessary.
So, if there are still D5 users out there dying to run it on SQL Server, here you go, enjoy!