Our joke from April first was not far from reality because Microsoft gave code to Mono and Xamarin continues the tradition of sidling next to Microsoft. Mono proponent SD Times says that Xamarim will contribute to Microsoft code:

Microsoft releases ASP.NET Web API as open source; Xamarin plans contributions

[...]

The Mono project is adopting code from Microsoft, such as the JSON engine and part of the ASP.NET Web Stack. That’s because Microsoft has released ASP.NET Web API and ASP.NET Web Pages (also known as Razor) as open source under the Apache 2.0 license. That’s in addition to ASP.NET MVC, which was already available to the open-source community.

Miguel de Icaza, CTO of Xamarin and founder of Mono, said that he is working to contribute back to the ASP.NET Web Stack, as well as adapt parts of Microsoft’s code into Mono. For example, he cited the JavaScript Object Notation stack. “As of today, we replaced our System.Json implementation (which was originally built for Moonlight) and replaced it with Microsoft's implementation,” he wrote in his blog.