From the researchers
Abstract: "New evidence for the geological origins of the ancient Delphic oracle (Greece)", Geology (2001), by J. Z. de Boer, J. R. Hale and J. Chanton. The important scholarly article; pay for full text.
"Questioning the Delphic Oracle," Scientific American (August 2003), by John R. Hale, Jelle Zeilinga de Boer, Jeffrey P. Chanton and Henry A. Spiller. Engaging, convincing article details the process of discovery, starting with de Boer's initial discovery of a fault, "but being aware of the classical tradition and unaware of the modern skepticism and debunking, he attributed no special importance to his observation."
Transcript of radio interivew of Delphi archaeologist John Hale, by Rachael Kohn of Australian Radio National program "The Ark" (8/8/2004). Good introduction to the oracle, with particular attention to the geological controversy. Hale also appeared on the ABC program Sunday Morning, but a full transcript isn't available.
News reports and summaries
"Fumes and Visions Were Not a Myth for Oracle at Delphi" by William J. Broad, New York Times (March 19, 2002). Also here (without sign-in). The latter site gives two other newspaper stories on the topic.
"Delphic Oracle's Lips May Have Been Loosened by Gas Vapors" by John Roach, National Geographic News, on new discoveries of faults beneath the temple.
About.com: Geology of the Delphi Oracle, by the geology guru Andrew Alden.
A Geologic Connection with the Myth of Apollo's Oracle by Erica Schneider. College paper reviews the issue.