D can access the C/C++ runtime libraries on a target environment and it also allows C programs to access its functions in a similar fashion.
[1] Is there a way to integrate the capability of using D programs on RIOT, as in binaries compiled by the D compiler?
Since RIOT is built with GCC by default, I would assume the required "D support" is already there - you need to enable the D programming language in your GCC (may require recompiling the GCC in some cases!).
However, GDC (http://gdcproject.org - the D language support in GCC) may not yet have support for your preferred target architecture. ARM is definitely supported - I use it myself on my ODROID (http://hardkernel.com) machines.
As pointed by @jpf (who is one of the GDC devs btw), you do not have the luxury of using the standard library, so you will have to use the C runtime all the time, which is not really a big problem...
If you do successfully run your D programs on RIOT OS, please write a blog article about the steps required to make it work as you wished. The D community will appreciate that.