Here's how it went down:
"I'm working on several games at once right now," John Romero said. "But [Brenda] had mentioned earlier that I'm working with a concept artist, and so I've got some cool imagery for the main character."
Brenda Romero jumped in: "I'm concerned about how much you're going to say."
"I haven't made a shooter since 2000. So I'm basically starting to work on another one," John Romero said. And the room. Went. Wild.
You can listen to the full episode when it hits Joystiq in a few hours. The new shooter is coming from Romero Games, rather than Loot Drop, the Romeros' social game studio. John Romero's 2000 shooter was Daikatana, which launched for PC and was published by Eidos Interactive. John Romero is of course a co-creator of Doom, the classic first-person-shooter from id Software.
Update: Romero's comments can be heard on the Super Joystiq Podcast Gamescom (Day One) Special podcast.