Actually, if you achieved Plat this year (2008), then your Plat elite status should be valid through 02/10. I do believe Marriott sends out the cards with the current year expirations, but you should receive another Plat card early next year that will be good for all of 2009.

My understanding is that even if you achieve an elite status through a challenge, it is treated the same as if you had the 75 nights this year.