-- As noted late last night here on the website, WWE stated during RAW that Brock Lesnar will answer Triple H's challenge for a match at SummerSlam on next week's RAW. It was not specifically said that Lesnar will be present though.
It should also be noted that a local advertisement that aired in the St. Louis, MO area was promoting an appearance by Brock Lesnar on the 1,000th RAW telecast. The event takes place on July 23rd from the Scottrade Center. The ad claimed Lesnar would appear on the show and asked "what part will be play?".
Other local advertisements that aired during RAW say Lesnar will be appearing on the August 13th edition of RAW. That is the week before WWE's SummerSlam pay-per-view.
As noted, Triple H vs. Brock Lesnar will headline SummerSlam.