I recently hit 75 nights this past Tuesday to obtain the Platinum status or so I thought. It turns out the Marriott Rewards system still does not have my nights recorded so I am still listed as Gold. I have checked into two different Marriott properties since reaching Platinum (according to my numbers), but am greeted at the front desk as a Gold. Oh well, maybe SPG, Hilton, or Hyatt has better programmers that can design a system that is faster to track activity.
Have your stay folios/points posted yet, just not the nights? If they haven't, call up Marriott Rewards customer service and check on the hold up. They might even be able to manually "push" Platinum status until your nights post.
I would also ask them to "adjust" your stay points to reflect the 50% point bonus with Platinum, and to add the 500 point welcome gift that you would have gotten from the hotel.