You can grab my solution at:

http://rubyquiz.com/cowsnbulls.zip

I didn't build a player, but did provide three different interfaces.

The files in the zip are:

     cowsnbulls.rb    -- The library that handles the game.   
Executing this gives a
                         command-line interface.
     tc_library.rb    -- Unit tests for the library.
     telnet_server.rb -- Quiz solution.  The protocol mentioned in  
the quiz didn't
                         appeal to me, so I just targeted Telnet  
instead.  This isn't
                         exactly a true Telnet server, since it reads  
line-by-line
                         (You won't see an answer to an "Are You  
There" command, until
                         you press return, for example.), but it's  
close enough
                         for this exercise.
     web_server.rb    -- Web interface through WEBrick servlets.   
(Just for fun.)
     english-words.10 -- The dictionary I've been using for testing.   
Part of
                         Spell Checking Oriented Word Lists (SCOWL)  
by Kevin Atkinson.

Enjoy.

James Edward Gray II