Running Windows 10 Tech Preview (build 9860) and Environment.Version returns 4.0.30319.0 all the time. In VS I can build and run .NET 4.5.1 apps but it still reports the .0 revision #. This may be a bug in Windows 10, I can't seem to do a repair on the framework, but due to that, I can't run #develop 5RC. The registry technique detailed in this MSDN article would work, except the release # is 381023 (which isn't listed in the table). The article also briefly mentions the Environment.Version revision #, so it seems that is considered an "official" way of checking it, but for whatever reason it's returning a "0" which is most likely what's causing #develop to not work.
Obviously this appears to be more of an issue with .NET or Windows than #develop, but this may be something you should be aware of. Perhaps the final Windows / .NET will have the correct revision information, perhaps not. Here's a screenshot that shows a .NET 4.5.1 app being debugged and the Environment.Version information, along with my registry keys, and CLRVer.exe results: http://i.imgur.com/S8ai4IZ.png
Thanks