Nearly Microsoft's entire application base are already written in C++. They would be foolish to rewrite even just the GUIs for the sake of implementing .NET and WinForms when what's there already works. It will take time, but I'm sure we'll see more WinForms
apps coming out of Microsoft going forward.
It took no time to port Quake II to .Net so why doesn't Microsoft do it to some of their popular applications?
Here's why I wouldn't use .Net WinForms for anything but intranet development: