We want to announce that there is a "quite new" (1 year old, but we hadn't discovered it until now) C# port of CEGUI called SharpCEGui available on bitbucket: https://bitbucket.org/icehole/sharpcegui.
Upon asking about the functionality, one of its developers replied to me that it in a good shape and can be used:
"Well the port it's in good state (there are some NotImplementedExceptions) but most of the samples are working.(the animation sample it's not ported) I have ported almost all the codebase of the original library except the new commits that You have made with the svg support and the commits of timotei for the new input handling but I will (lack of time)."
The developers are part of the company Icehole Games (http://iceholegames.com/), if you want to check them out.
We haven't personally tried the port out but it looks very promising. If you have tried it out and want to tell us how it is, then feel free to discuss the port in our CEGUI forums.