A Mono developer should avoid looking at the .NET code at all costs. That would only set them up for patent law suits.

Unlike GPL2, the Apache license has an explicit patent grant. When I first saw this announcement, I didn't realize what effect it will have on the patents used to bring up any chance they got: they join the patents already available under various approaches (due to C# standardization or the Communit Promise). And I believe you can now avoid the goofiness of all end users explicitly needing to accept the Community Promise.

The .NET reference source is licensed in a way that Mono developers can look at it without scratching their eyes out afterwards.