The T&C's read:
Marriott Rewards® members who register for this promotion by June 30, 2010, will earn 25,000 bonus points after 20 paid nights, plus an additional 15,000 bonus points after 25 paid nights – up to a maximum 40,000 bonus points, made between June 1, 2010, and August 31, 2010, at a participating Marriott® property.
There is no mention of stays needing to start or end within the time period like it did in earlier blunders by MR. Check your Promotion Central page after the stay posts and call if it does not include the appropriate nights.