MDAC is a two side component. i.e; on infrastructure (windows) side and driver (sqlserver) side. MDAC is replaced by SNAC. SNAC is on the "drivers" side.
In a nutshell, SQL Native Client is a stand alone data access Application Programming Interface (API) that is used for both OLE DB and ODBC. It combines the SQL OLE DB provider and the SQL ODBC driver into one native dynamic link library (DLL) while also providing new functionality above and beyond that supplied by the Microsoft Data Access Components (MDAC). SQL Native Client can be used to create new applications or enhance existing applications that need to take advantage of new SQL Server 2005 features such as Multiple Active Result Sets (MARS), User-Defined Types (UDT), and XML data type support.
MDAC now ships as a component of the Windows operating system and as such there are a number of setup, redistribution, and deployment issues that have occurred as a result of this. you have developed an application based on the latest MDAC release only to discover that when you deploy it, the users in your organization do not have the latest MDAC release so your app won’t function correctly. By wrapping the OLE DB and ODBC technologies into a single library, we are able to avoid these issues by making a clean break from MDAC so that you can effectively deploy SQL Native Client as needed, without concern about if it will “play nicely” with other versions of MDAC.
Data Access Technology Road Map
http://msdn.microsoft.com/en-us/library/ms810810.aspx
