Forgot to blog about this when it happened, but since I've gotten a couple of questions lately...
In February, we released a new version of the Gtk# for .net installer. It packages Gtk# 2.12.8 and the associated gtk+ binaries, including the registry keys necessary to build projects with MSBuild and add references using the references dialog in VS.
This single installer combines the managed development functionality previously provided by both the SDK and Runtime installers in a significantly smaller download than the old SDK installer, since it no longer ships many of the C development files packaged in the old installers. This installer also works properly on XP and Vista, unlike previous versions.
In the near future, we will be announcing the availability of additional installers supporting the use of mono libraries like Mono.Posix, Mono.GetOptions, and Mono.Addins on .Net and maybe a new application installer or two as well. Stay tuned.