Starting with Visual Studio 2008, you'll be able to use the Visual Studio 2008 Shell as the starting point to create your own development tools. The Visual Studio Shell will be available as part of the Visual Studio 2008 SDK, but I believe we'll have to wait until VS'08 RTM's before we'll be able to play with creating apps using the shell. On the Visual Studio 2008 Shell page, there's a screenshot of Floorplan designer built using the VS'08 Shell.
This looks like a really neat feature that'll allow a much quicker development of tools that can utilize the VS Shell as their base.
I guess this'll bring a new meaning to "all your base are belong to us". :)