Hello, With OpenSuse (at least since 10.1), you've got a Mono runtime which you can choose to install during distro's own installation. But I don't think that is the real concern, since an installer is provided for all distros. But it looks like the most supported one (by Mono project itself) is actually Suse. You even have a VMware image.
Cheers, Pierre -- "Mon horizon était désormais les pétales courbés, Ma nouvelle demeure resplendissait de flèches d'herbes folles Et d'oreillers de pollens odorants." (Naïmah, le philosophe Fémereif)