On February 22, 2010 the new law allowing citizens to carry firearms in national parks in accordance to state laws and procedures went into effect. Read the NRA-ILA Update here.
More info and list of parks per the National Park Service:
http://www.nps.gov/nero/firearms/PA/index.htm
Parks:
Allegheny Portage Railroad National Historic Site
Delaware Water Gap National Recreation Area
Deshler-Morris House
Edgar Allan Poe National Historic Site
Eisenhower National Historic Site
Flight 93 National Memorial
Fort Necessity National Battlefield
Friendship Hill National Historic Site
Gettysburg National Military Park
Gloria Dei Church
Hopewell Furnace National Historic Site
Independence National Historical Park
Johnstown Flood National Memorial
Steamtown National Historic Site
Thaddeus Kosciuszko National Memorial
Upper Delaware Scenic and Recreational River
Valley Forge National Historical Park