Already done in a few languages:

http://www.reddit.com/r/dcpu_16_programming/

I think so far there are only assemblers, not C compilers (or any high level languages)?

Ahh, I was only answering the last question.

Now that we have some emulators, people are working on compilers.

Very good subreddit but, I meant physically assembling a DCPU :) If not physically, with verilog or vhdl at least