Word is that Brock Lesnar's upcoming match against Triple H against SummerSlam will be a one-shot deal, with no rematch planned. At this time, the plan is for Lesnar to win and get his heat back following his loss to John Cena at Extreme Rules.
The match between Alberto Del Rio and Sheamus for the World Heavyweight Championship has yet to be made official for SummerSlam. It was said on RAW that Del Rio wouldn't wrestle until he gets his World Heavyweight title match, and it's said that if that's true, we wouldn't see him in a match until SummerSlam. While Del Rio will likely be in the title match, WWE hasn't announced it yet but were speaking as if they were.