Quoting Larry Garfield larry@garfieldtech.com:
Drupal currently cannot connect to more than one database type at a time; All databases must be either MySQL or PostgreSQL. While one could write an MS SQL driver for Drupal's current database implementation, there's no guarantee that it would work with the vast array of contrib modules out there; in fact, odds are against you. MS SQL, because AFAIK it doesn't require any additional field handling, is one of the few that are even possible to think about. You would have to move the entire Drupal system over to MS SQL, though.
Well I suppose that the fact that I connect to multiple databases using Drupal makes your point somewhat mute but both databases were MySql though I see no reason why one couldn't be a different DB engine. You use an array of databases in the settings.php file to give the connections and you use db_set_active[1] before performing the queries to the second DB and then after to reset back to the default. There is nothing suggesting to me that the database connections must be the same.
Siren is of no use in practice, because it makes far-reaching changes to the syntax of every single query, changes that no contrib module has or will ever implement.
I only suggested that it could be used as a starting point for the MSSQL engine. I agree that it the changes are too far fetched to be used wholly.
The database layer is being gutted and rewritten for Drupal 7 to address many of these issues; I know that won't help you now, but we are aware of them. :-)
Yes, PDO will help greatly.
[1] http://api.drupal.org/api/function/db_set_active/5
Earnie -- http://for-my-kids.com/ -- http://give-me-an-offer.com/