So I was excited to see that the DLC that was included with all new game purchases was still inside the case, basically untouched, so I was all happy thinking how I just made out on the game and $15 DLC for the price of the DLC alone, and then I turned the case over, scoped out the back screen shots and such and then I noticed, low and behold down on the bottom left side of the case in small print it says "Includes: Downloadable character and quest. A $15 value. One-time use code available with full retail purchase. Expires April 30, 2010." 

I don't want to get your hopes up too much, but I have read somewhere (although it may well have been wikipedia, so take from that what you will) that the code does in fact still work. Or at least it did for some time after April 30. 
 
You may as well attempt to use it anyway, nothing to lose I guess.

Yep, companies do that. When you buy Blu-Rays and it comes with a digital copy as advertised, it expires about 3-6 months after release. I happened to pick up Speed Racer on sale at Wal-mart. Of course it says the digital copy is included. Well turned out it expired a long time ago. Good thing it came with a DVD copy. I ripped the hell out of that. 

Whew, thank god I pirated it months ago! 
 
More seriously though, Really? Expiration date? So now there is no reason I should buy it new.

Good on them. They weren't getting your money with the used purchase anyway. 
 
If you like the game, pay the developer for it. In this case you can do it by buying DLC.

Sort of negates the idea of it being about buying the game not-used and makes it seem like what they're really after is you buying it when it first comes out.  Though if it is still working there isn't much room to complain.