I'm releasing the first Gtk# 2.12 installer for Windows today. There has been a lot of community demand for this, so I wanted to get it out. However I do want to be clear that this should be considered an EXPERIMENTAL installer by Medsphere's standards. It has not gone through any QA process, and Medsphere will absolutely not be providing any official support for this.
That said, I'd be happy for people to use it, abuse it, and send bug reports to me regarding either the installer itself or for GTK+ on Windows in general.
If you're interested in using it, you can grab it from: http://bzr.medsphere.com/~cody/installers/gtksharp-sdk-2.12.3-1.msi