Will Unity support .NET 4.0 framework in the near future?
There are no concrete short term plans. At some point we defenitely will support it, though a timeframe is hard to give right now.
This is the roadmap of Mono:
http://www.mono-project.com/Roadmap
You can see that .NET 4.0 is still under development. I don’t know if Unity always comes with the newest version of Mono though.
To answer the question literally: Unity will never use .NET, since .NET is Windows-only.