I started with a tiny, flat/slot jeweller's type screwdriver along the front bottom edge and tried to not pry into the outside of the shiny piece.  I wasn't 100% successful in not marring the shiny part.  It's not easy to get apart but the bottom of the shiny comes away from the not shiny the easiest I think.  The hard part doing it the way I did was the top/back of the drive.  I ended up not using a screwdriver to pry at the edge/seam of the back side.  I stuck a screwdriver between the drive itself and the back edge to kinda work that part up - starting at one end.  The interlocking plastic pieces are kinda like one long ridge on one piece and then fingers that grip both sides of that ridge on the other piece.  I think you can kind of see that from the pictures.  The backside bottom piece has the ridge and the backside top piece has the finger like pieces.  On the fron it's reversed, the bottom has the fingers and the top-front has the ridge.  

\ /   = finger like part
 |     = ridge
\|/   = what it would look like if we could see them after it's put together.

That's probably not exactly how they work but it's close.  I have here the original size crop pics and notated the front and back sides... might help.  I can't do really good macro shots these were as close as I can get with my camera - the files were all lost so the links below are worthless now.

http://videosean.gotgeeks.com/i/WDPassport_120/notes/DSC06203n.jpg
http://videosean.gotgeeks.com/i/WDPassport_120/notes/DSC06204n.jpg
http://videosean.gotgeeks.com/i/WDPassport_120/notes/DSC06205n.jpg
http://videosean.gotgeeks.com/i/WDPassport_120/notes/DSC06207n.jpg