Channel 9 has now the video recordings of the talks that were given at the 2009 Developer Tools Ecosystem Summit:

http://channel9.msdn.com/tags/Development-Tools-Ecosystem-Summit/

I’d suggest to start with the keynote:

VSX100: Visual Studio Extensibility Keynote: Past Present and Future
http://channel9.msdn.com/posts/VSIPMarketing/VSX100-Visual-Studio-Extensibility-Keynote-Past-Present-and-Future/