One of the quotes I picked up on from this recent article on eWeek was "...Microsoft is really putting lots of effort into making Visual Studio .Net a third-party platform..." This is certainly true and I think a platform is really the right way to think about extensibility for Visual Studio. There are a lot of parallels you can think of between VS and an OS like Windows. For example:
This is the overall vision that we have for Visual Studio going forward. With this model in mind, you can think of the current COM-based VSIP interfaces as being parallel to the Win32 API and the Helper classes (called the “Managed Package Framework” in Whidbey) as being parallel to the .NET Framework on Windows.