Figured it out, there are about 6 different promotions, in three different groups for the “Unexpected Bonus” promo:

1. Category 1-5 Free Night Certificate After Two Stays (Max 2)
2. Category 1-4 Free Night Certificate After Two Stays (Max 3)

3. 25K Bonus Points After 15 Nights + 10K Bonus After 20 Nights
4. 25K Bonus Points After 15 Nights + 2K Per Night For Additional Nights (Max 45K Bonus)

5. 35K Bonus Points After 20 Nights + 10K Bonus After 25 Nights (Max 45K Bonus)
6. 35K Bonus Points After 20 Nights + 2K Per Night For Additional Nights (Max 55K Bonus)

People are reporting that they’ve had success calling Marriott and switching between promos in the same group. This means you could call Marriott and switch from the 3 nights at a Cat 4 to the 2 nights at a Cat 5 property. Just depends on your preference, quality vs quantity.

More details here: http://www.flyertalk.com/forum/marriott-rewards-including-ritz-carlton/1461486-summer-2013-megabonus-unexpected-bonus-3.html

That’s odd…. this is the link I used below.
https://www.marriott.com/rewards/promotion.mi?promotion=MY13&nt=CJ&co=US&aff=MARUS&affname=6150572

To earn your free nights:
Register by July 31, 2013
Stay twice at any participating hotel between May 15 and September 2, 2013
Earn one free night after each second paid stay, up to three free nights
Enjoy your free nights at any category 1-4 hotel worldwide
This bonus season, expect big things—and earn even bigger.

Your link tells me the Cat 1-5
https://www.marriott.com/File_Blocks/MarriottRewards/PromotionCentral/Terms/2013/MY13/US/MY13A_terms_us.html

But…. when you click the link with in it registration says cat 1-4
http://www.marriottrewards.com/UnexpectedBonus

I was really hoping it was Cat 1-5 though

You have a link for that? I just doubled checked the T&Cs here: https://www.marriott.com/File_Blocks/MarriottRewards/PromotionCentral/Terms/2013/MY13/US/MY13A_terms_us.html and its still showing category 1 to 5.

Marriott Rewards members who register for this promotion by July 31, 2013 will earn one (1) free night award at a category 1-4 hotel after each second paid stay made between May 15 and September 2, 2013, at a participating Marriott property.