I just managed to connect a Linux machine to a MS SQL Server 2000, and oh, what a trip it was!
Bottom line: FreeTDS (configure the server as using TDS 4.2) and DBD::Sybase. If you need to do this, look no further.
As usual, mpeppler rocks when it comes to Sybase, I found the solution in a mailing list post from him.
Aparently Microsoft broke the ability to use the Sybase Open Client libs in SQL Server 7. Great, thanks a lot! That would have been my first choise, considering it's already installed on our servers.
A waypoint until I reached FreeTDS: ODBC, but the only viable solution pointed to FreeTDS anyway, so, like, why not use DBI?
Interesting article: Why Isn't ODBC A Standard Feature Of Linux?