This post from Victor Garcia Aprea (VGA...I wish MY parents had been more forward looking when they named me, although I could have ended up with a crappy acroname like JDBC or something) struck a chord with me. Extending some parts of Visual Studio is just much much harder than it should be. I've had some success in the past, but it was always extremely painful, and some cool things never saw the light of day because of it. if VGA calls this stuff hard (AFAIK he works with the PnP & VS SDK team and works for Clarius who made the T4 editor, IronPythonStudio and a bunch of other VS-related things) then what hope do the rest of us have?
So far I know of two efforts to create a new developer shell - nautilus (only known shipping incarnation is Expression Blend), and now emacs.net. Considering the weight of history the current VS shell has I doubt the others will supplant the memory-lovin' COM beastie we know as Visual Studio.