Some assembly is required for that.

MonoTouch is currently based on Mono 2.10, while C# 5.0 is based on the new codebase on Mono 2.11.

We are waiting for Microsoft to officially bless C# 5.0 as complete before we can ship it, otherwise we will break people's code.

The current version of Mono4Android and MonoTouch do not support async/await yet, but that is on the roadmap (once Mono 2.12 [currently in beta] is released).