Most core language features are usable, but there's no Phobos port and if you need to import functionality from external DLLs, you'll have to hand-write some import files (following the model in
druntime/import/System.di); I hope to get around to writing a tool that automates the process one of these days -- and it will most likely be in D.
There is a Visual Studio 8 (works with Visual Studio 8 Express) solution file in the source tree, and there is also a Makefile for the adventurous. The project compiles on Linux but it does not work quite well with Mono -- I strongly suspect it is Mono's segfault.
Check it out at http://dnet.codeplex.com/!