Apparently deep in the jungle in Vietnam. More pictures at the story.
At 262-by-262 feet (80-by-80 meters) in most places, the Son Doong cave beats out the previous world-record holder, Deer Cave in the Malaysian section of the island of Borneo.
Deer Cave is no less than 300-by-300 feet (91-by-91 meters), but it's only about a mile (1.6 kilometers) long.
By contrast, explorers walked 2.8 miles
Update 7/28/09: Nellisir points out the Sarawak chamber, may actually still be the biggest cave chamber:
You could fly dragons around in these things!