I just saw an announcement for this project on freshmeat.net and thought
I would mention it here:
http://lrnj.com
This fellow is developing a computer role-playing game (CRPG) with the
goal of “… [making] Japanese accessible to people who enjoy anime or
video games, and wouldn’t otherwise learn it”. I am just the kind of
unmotivated individual that the author is after 
Have not had a chance to download and try it out myself. It is not open
source, appears to be shareware. Available for Windows and Linux. So
there you go.