I just got a promo from Marriott to earn 10K DL miles for every 3 stays at Marriott properites. It defines the question above and counts multiple hotels in once city as one stay. I have not found this to be the case with Hilton, espcially if you switch hotel types like DT, to HI. So far I think the information posted on this thread has been correct, but I am not sure if you have to physically check out each day or not.