While Tony Hawk's Pro Skater HD combines the first two games in the franchise in one high-definition mash-up, Activision has already announced plans to release content from THPS3 as DLC. Airport is one of the few confirmed levels to be released as DLC "around 30-45 days after launch."
In addition to new levels, the $5 add-on will include one of THPS3's most important gameplay tweaks: the revert, which lets you string together numerous combos as you go from one side of the level to another.
According to Joystiq, revert will only be included within the DLC and can't be used in the base THPSHD game. Why? "Reverts were introduced in THPS3 and the levels were designed accordingly."
Tony Hawk HD will be available on Xbox Live Arcade on July 18th as part of Summer of Arcade. A PS3 release will follow shortly thereafter.