Some time ago I've downloaded the add-in for VSIP 2003 SDK and was surprised. Microsoft eventually has published its MSSCCI interface. Here is a quotation from documentation that I had installed:

The Source Code Control (SCC) SDK provides resources that enable you to add source code version control capability to the Visual Studio .NET 2003 Integrated Development Environment (IDE). It allows you to plug in any SCC DLL that complies with the SCC API outlined in this documentation. The IDE handles the UI for common SCC operations and defers to the SCC plug-in for some advanced source code control features. An SCC plug-in implements functions to perform SCC operations. The SCC SDK documentation includes installation guidelines, a detailed specification of the SCC API, a skeleton sample of an SCC plug-in, and a test guide to verify integration and operation of the SCC DLL.

So, from now, everyone can download VSIP 2003 SDK Extras, read “Source Code Control SDK” topic and then implement this interface (I mean MSSCCI). Hope this will simplify lives a lot of developers.