Running Nant -v
on mono gives informs me it uses gmcs.exe (C# 2.0).
Starting '/Library/Frameworks/Mono.framework/Versions/2.8/bin/mono ( "/Library/Frameworks/Mono.framework/Versions/2.8/lib/mono/2.0/gmcs.exe"
How to setup Nant to use dmcs.exe (C# 4.0) on Mono?
As is asked and answered here, the newest nightly build supports the mono C# 4.0.
I found that mono-4.0 is not supported for newest version of Nant (0.91 Alpha).