plug-n-play MS SQL Server replacement

I've been using PostgreSQL for a few months now and am very happy with its
functionality and performance.  Top-notch product that competes favorably
with every DB I've used.

However, most of my tables are still in MS SQL Server 2000 Enterprise.  I
often have to make it my target for purposes of compatibility.  On some
applications, the license costs are killers at around $400/client.

So, here's my thought.  How difficult would it be to build a server daemon
that used PostgreSQL on the backend, and presented it to the network as a
SQL Server compatible DB?

Some of the work is already available in the 'tsql' client.

This must've been thought of before, but does this idea have legs?