I don't think doing it for the main tree of .net would actually be very helpful. .net is already completely free as in beer, and is tied to a closed source OS that isn't going anywhere on that front. Very little about .net that isn't already open source through Microsoft(the CLR) or the Mono project isn't dependent on closed source stuff, and it probably does expose some proprietary knowledge about the OS that Microsoft would rather not let out. It might help Mono keep up with .net a little bit more but I doubt it would be as noticeable as you would think, the difference between having a solid specification + a blessing IP wise (which MS already provides) and the source code when you have to change the code significantly anyway is nil.