This actually depend on the compiler (and profile/runtime) not the Mono version. IOW some things might work on Mono 2.10 and not with MonoTouch 6.x.
Current versions of MonoTouch ships with the smcs
compiler and a 2.1 based profile (mostly Silverlight and some .NET 4.0 additions).
Newer features, like covariance, requires a full 4.0 compiler (e.g. dmcs
in Mono 2.10 or mcs
in Mono 3.0) and runtime.
Future versions of MonoTouch (likely this spring with alpha/beta in early 2013) will provide a 4.0/4.5 runtime and compiler.