There are in reality four levels. Platinum Premier is the other level and it is a addition to Platinum, perhaps a lateral award?
In the past folks who stay over 125 nights per year for three consecutive yearsmay be given this status. It is a changing thing--you cannot apply, it is given by HQ at Marriott to those who are in the top two or three percent of Marriott stayers. With the downturn in the economy there may be a reduction in the 125 requirement but only HQ knows what's what.
There is also Platinum Emeritus (Lifetime) which is a recognition of 12 years or more in the program (one of which has to be at the Platinum level), 1000 paid nights stayed and three million MR points accumulated over the life of your membership.
Both Premier and Emeritus put you first in line for perks, if there are perks to be had, when you check in. Premier has this year (2009) a year-long, one-night upgrade when using reward points. And a special number to call for assistance. That phone is answered by specially trained Platinum folks who I have found to be very helpful.
Hope that answers the question