Yes, definitely you are suppose to get one, although sometimes they do forget. If you request one, they will definitely send you one...it happen to me before.
Congrats! on your very handsome backpack!
I have a Prada messenger bag that I bought at Prada in London two years ago and they didn't give me a dustbag... they said that they only came with some bags :s
this is what i was told too, that the nylon prada backpack don't come with dustbag or cloth dustbag. when i bought mine a few years back from the boutique, it just came in a plastic drawstring bag. i am not sure if that has change.