Found it. It has to do with the Nvidia drivers, not with Arma or DayZ. In the Nvidia control panel, there is an option to disable the message, which apparently applies to all games in the current driver/application version to date.
After starting the game in Quad-SLI mode, I see a vertical banner, which appears to be a usage metric, and the words QUAD SLI on screen at all times. Presumably, it's a debug feature. How do I hide that? Thanks guys.