In one of my last posts I said that the .NET source code is available. While this is still true, you might have seens some trouble in getting the source, because the source is only available on demand while debugging a component. Also you must setup VS to retrive the source/PDBs.
But if you just want to look at the source, there was no easy way to do this.
On CodePlex there is now a tool called NetMassDownloader, which allows to download the complete source (or only from indivudual DLLs) to a directory.