Checkout Marriott's FAQ for the new changes to the Rewards program:
Marriott Rewards 2009 Program Enhancements FAQs
Excerpted from their FAQ:
"If I redeem for 10 nights, do I get the 5th and 10th nights for free?
Yes, when you redeem for 10 nights you will get the 5th and 10th redemption nights for no additional points."