What is the data provider for nhibernate? Is it ADO.NET?
Take a look at http://community.jboss.org/wiki/DatabasesSupportedByNHibernate.
It shows what tools/drivers are used to connect to different databases. for example:
Oracle 9i and 10g are supported, both using Microsoft driver (System.Data.OracleClient) and using Oracle driver (Oracle.Data.OracleClient).
Or:
To work with Firebird, install the latest Firebird .NET Data Provider.
I'm not sure whether it uses ADO.Net for SQL Server connections under the hood, But it seems that it doesn't use ADO.Net for other database systems.