[quote name='Survivalism']Seems to me like that's something you make sure of, you know...before you leave the store.[/QUOTE]
It's the responsibility of the salesperson to put the disc in the box so it really shouldn't be necessary to double check.
In any case, OP, you should be fine, I've had this happen plenty of times with no troubles. Just go to the store, explain your situation, and they shouldn't take any kind of issue with it. Hell, I had two weeks go by once before I noticed I didn't get the disc, and it still worked out fine.