I've been waiting this for a long time. There's also TinyRB (http://code.macournoyer.com/tinyrb/) but it's too small subset of ruby to be useful and seems abandoned.
But my first impressions are not very positive (stripped x86_64 executables with -Os):
-rwxr-xr-x 1 user group 708480 Apr 20 11:27 mrbc
-rwxr-xr-x 1 user group 713056 Apr 20 11:27 mruby
-rwxr-xr-x 1 user group 712128 Apr 20 11:27 mrubysample
compared to lua-5.2.0: -rwxr-xr-x 1 user group 147864 Apr 20 11:28 lua
-rwxr-xr-x 1 user group 99168 Apr 20 11:28 luac
I'm not a big fan of lua as a language but it has been the only option to do scripting on small embedded systems. I've manged to compile size-optimized version of lua on arm to the size of about 80kb but mruby just might be too big for my embedded usage.