From the release notes:
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OSX support debuts with a native Quartz backend. This means that there
is no need to run your applications inside X11, although you still
need X11 installed as we use FreeType and font-config that are part of
the X11 distribution on OSX) [Geoff, Will Johannson, Attacom ]. This
is at this point a preview and not yet fully supported, but many
applications now work.
To use Windows.Forms on MacOS, you should use the macpack tool to
"wrap" your Windows.Forms application properly (so that users can
double-click on it).
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Also, from Miguel's blog
http://tirania.org/blog/archive/2007/Dec-02-2.html
Basically, Mondevelop and GTK# will finally be native and packaged for
OSX!