I'm using D for my research work in the area of computer graphics. I and others have had papers published in our fields based on work done using D. I think it's definitely ready for use on small to medium sized research projects where performance matters. It's a nice fit for research work because often you're starting from scratch anyway, so you don't have much legacy code to worry about integrating with.
Another popular area for use seems to be web services. Hopefully someone else can comment who's in this space, but there too I think the idea is that performance often really matters so you want a compiled-to-the-metal language. Services are often fairly small, self-contained processes, so interop with large amounts of legacy C++ code is not really necessary or useful. Thus D can get its foot in the door.
I think D will continue to gain grass-roots followers in this way -- on smaller projects that for whatever reason can afford to ditch the C++ legacy in order to gain a programming language that's much more enjoyable to use, and perhaps more productive too.
But until there's a huge number of grass-roots users there won't be much in the way of big corporate users I suspect.
