I have to admit I do not know much about the Rainbow system other than it is a Content Management System written in C# with versions for ASP.NET 1.1 and ASP.NET 2.0. It definitely bears investigation to see how it compares and contrasts with DotNetNuke.
I am definitely interested in seeing how easy (or not) it is to integrate Subtext into another system. One thing I have been working on when I have time is to refactor Subtext to clean up some code and make it easier to understand the source. As a team, we also hope to make the codebase even more modularized. This stuff is fun.