There is no official word on this. However if you consider that:
- C#/mono is available on android
- C#/mono is available on ios
- C# is available on windows phone 7 and 8
If ubuntu was to prevent use of c# it would be more restrictive then all its competitors.
There are a few issues to overcome first however, firstly c# binding for QT although available have not been packaged for ubuntu (yet). There is currently a blocking issue on mono for arm in debian which needs sorting out. This shouldn't be a big issue as mono works on IOS and android both of which are arm.
My guess is that ubuntu wont support it, however they also will not prevent it. I fully expect that you will be able to develop using c# for ubuntu phones and tablets.