Looks like there isn't going to be an easy answer dealing with the Visual Studio 2005 SDK distribution EULA. I guess we'll see what happens when VS2005 is RTM, but honestly, I just saw one screenshot of the Resharper IDE from JetBeans...
JetBean's IntelliJ allows for open source plugins, so I can say with strong hope that the Resharper IDE will, also.
That's probably where I'll set my sights for a real IDE-integrated Boo addin. Its true that I could use Java and do one for Eclipse or X-Develop, but to be honest I want real .NET integration. Automatic adding of web references. Automatic winforms designer. ClickOnce deployment.
All that stuff is only going to be possible with a "real" .NET IDE, rather than me having to recreate all that from scratch because the host platform didn't support it.
The guys at JetBrains seem really smart, so the API might be just what I'm looking for... managed and loving it. ;)