Well this is just my educated guess. In Longhorn, if it's not low level (You can't "see" it) - it's written in managed code. It doesn't really matter whether the code was written in managed C++ C# or VB as the executable will be quite the same. For parts
where interacting with driver/kernel directly is not required, I think the most common language will be C#. Avalon, Indigo etc - the new things will be definetely 90+% C#. The legacy stuff where existing code from Win2003/XP etc is used, is still C++.
I wonder if there is few/a lot of existing C/C++ codebases they have decided to "/switch / port" to managed C++ though - like the Quake 2 example.
w3bbo: "From what I've seen, its still unclear with Longhorn will contain DotNet"
Better question is, is the .NET somehow more closely tied to Longhorn than what it is in XP? Hard to say, but you can be sure that the framework is always in the memory - most likely even when the user has not even logged in yet.
Beer28: "I don't know of any operating system".."that is partly written in managed code."
I think this depends on what pwzeus meant by "Longhorn". For me Longhorn is the stuff that we do not have in XP/2003 currently. For the _new_ parts that will be the Longhorn experience it just makes no sense to use unmanaged code. But if we talk about windows
in general, I would believe that in Longhorn the majority of code overall is still inherited from previous Windoses and what parts of that is /switched to managed is uncertain - not many I would presume.
pwzeus: "or in which environment"
Based on what you can see in videos, blogs etc - I believe that different MS devs get to use their favourite coding environment where it is not required that everyone uses a particular one for some technical reason.
So the answer would be most certainly atleast Emacs and Visual Studio.