WWE United States Champion John Cena appeared on Tough Enough last night and seemed to accept Seth Rollins’ Title vs Title SummerSlam challenge. This was later confirmed on WWE.com’s Summerslam page, so the match is definitely going ahead:
"Amid speculation that the Cenation leader would not be cleared to compete at The Biggest Party of the Summer, Cena accepted (with Daniel Bryan's support) Rollins' challenge during an appearance on the Aug. 11 edition of WWE Tough Enough."
So this means Seth Rollins will be putting the WWE World Heavyweight Championship on the line and Cena will be putting the US title on the line, with the winner taking home both championships. It’s yet to be seen whether this could lead to a unification with the US belt being dropped.