I was curious about the 2 side referral too, I think what it means is both the referrer and new user get 250 MB's of additional storage.
I specifically didn't use their service because I think that going out to tell people "you should use this service, it is great and free" and then profit from it wrong (I don't have a problem with money but I want to keep my money and friends separate).
Also I think its a very silly reason not to use Dropbox.
Sorry for the confusion.