If you've recently installed the SQL Express version of SQL Server 2005, you may or may not have noticed SQL Native Client (SNAC). If you want to use the new features of SQL Server 2005 like multiple active resultsets or snapshot isolation from OLE DB, ADO, or ODBC, you're going to need SNAC.
SNAC is meant to separate SQL Server's OLE DB provider and ODBC driver from MDAC. Currently the SQL Server OLE DB provider (SQLOLEDB) and ODBC driver ship as part of MDAC (Microsoft Data Access). A new version of Microsoft Data Access (MDAC 9.0) was planned in order to support the new SQL Server 2005 functionality. MDAC 9.0 won't happen. Instead, the current components of MDAC (2.8 SP1?) will ship as part of the Windows family of operating systems. MDAC will change infrequently, changes will not be tied to new SQL Server functionality. SNAC will be versioned with new versions of SQL Server and ship separately.
Once you've installed SNAC, you should see a new OLE DB provider (SQL Native Client) and a new ODBC driver (SQL Native Client). You need to use them instead of the older versions to get the SQL Server 2005-specific functionality. Note that they need to be coded into the connection string (OLE DB, ADO, or ODBC) or ODBC DSN. The SQLNCLI provider does not support XML like the SQLOLEDB provider does.
If you're using ADO.NET's SqlClient data provider, it has been rearchitected so it doesn't use MDAC. Or SNAC, as far as I know. If you've used SqlClient with SQL Server 2005 beta 1, you won't need "USE MDAC9" in connection strings as MDAC 9.0 isn't shipping with .NET 2.0 beta 1/SQL Server 2005 beta 2. SNAC is provided for vendors that still use OLE DB, ADO, and ODBC in the products. Nice feature. I have a soft spot in my heart for OLE DB (having taught provider writers for a few year), and having MDAC as a part of the OS is overdue. Not shipping MDAC for SQL Server changes will be a good thing.