Here. "Register to become a Visual Studio Industry partner and get FREE access to the VSIP SDK. As a VSIP you will have access to the technology, private newsgroups and secured technical content to assist you in seamlessly integrating your tools, components, and languages into the Visual Studio .NET IDE."
Everything you always wanted to do to integrate seamlessly into VS.NET used to cost money. As of todays, it's free! Wahoo!