I had pre-ordered already, but just got my code yesterday. So I canceled the old pre-order, and pre-ordered again using the code. The good news is that it accepted the code. The bad news is that it didn't apply the $5 discount. So, I have an email into support asking that they take a look into it.
I just hope that me canceling one pre-order and re-ordering with the discount code doesn't screw things up with their system somehow.