there was one earlier this year with up to 40k points and there is currently a promo running for up to 25k points I believe. Check out the insiders lobby area for more info or your promotion central link from your account.
There is a current promotion called "Marriott Rewards Bonus". As a platinum member I believe you should have been auto-enrolled. Log on to marriott.com and check Promotion Central under your account. There are two (possibly more) offers for the current promotion which they assign to you based on some sort of algorithm. One of the offers is make two stays earn a free night up to a total of two free nights. These are the standard Category 1-4 certificates. The other is a nights/points promotion. Here are the details: Register by June 30, 2011. Stay at any of our participating 3,400+ participating Marriott brand hotels worldwide between June 1 and August 31, 2011. Earn 25,000 bonus points after 20 paid nights + an extra 15,000 after 25 paid nights. If you are enrolled in one and wish to switch to the other, you can call Customer Service and they should be able to put you in the one you'd prefer.