Transact SQL compatibility layer

Is there such a thing as a translator/proxy that can pretend to be a
Microsoft SQL server and proxy all the commands to a PostgreSQL server,
and the results back again? Obviously the purpose of it would be to
allow an application written for MSSQL Server to work with PostgreSQL
without modification.

If not, has such a thing been considered? What would be the obstacles to
developing such a proxy?

Thanks

James

(Not subscribed to the list at the moment - I'll check the archives but
please cc me on a response)