I finally was sucessful compiling Mono 2.0.1 and Gtk-Sharp 2.12.5 for the Neo Freerunner.
It is tested on OM 2008.12.
The binaries are found at: http://cid-2c33e5173bc28fca.skydrive.live.com/browse.aspx/.Public/mono-2.0.1
For a simple full install download all opk’s or the zip- Archive with all of them and copy them on the Neo.
In a shell execute the following command:
ls *.opk | xargs opkg install $1 -nodeps
So all packages will be installed and there should be no problems with dependencies.
Here are some screenshots of an Gtk# application running on the Neo. It shows my PIM Sync tool for the Neo: