I've been fiddling with WPF a little (as in, about three hours) and it does look like it might be suitable for doing sublime-like supplemental programs. Here's my current attempt at a sublime-style window;
Two things strike me;
1) It should not be too difficult to create apps which are 'satellite' to Sublime and which are launched from Sublime commands. I'm going to experiment with a file and folder tool which will load when Sublime loads and which monitors Sublime's open files. I may also be able to put together a library of sublime-like controls for WPF. If so, any other .Net developers out there may be able to use it to create dialogues and forms.
2) If Jon could host WPF controls (and I have _NO_ idea how hard this would be) then we might be able to have a panel which contains a WPF control. This would mean that someone other than Jon could develop some of those things that people are looking for -- like project management.
As yet it's very early days, but WPF+IronPython might be a good basis for some package development.
(EDIT: I should be clear. WPF is the Windows Presentation Foundation, the GUI system used by Intellipad and the newest .net GUI system. Intellipad also has a python-based plugin system, which I assume is IronPython)