In the course of talking to former Xbox exec Ed Fries about Atari's 1974 racing arcade game Gran Trak 10, former Atari engineer Ron Milner offhandedly mentioned of what now seems to be the earliest example of an Easter egg in an arcade game -- Atari's 1977 space game Starship 1.
This is a big deal because until now, the Atari 2600 game Adventure has been popularly recognized as the earliest known example of a video game shipping with an Easter egg from a dev hidden inside.
Fries went on to verify that yes, Milner's Starship 1 trick -- which involves holding controls in a certain sequence when the game boots to give the player 10 extra lives and a "Hi Ron!" message -- is real and does work on an authentic Starship 1 machine.
In a post on his blog, Fries extensively documents his process of discovering the Starship 1 secret and acknowledges that while there may yet be earlier examples of video game secrets waiting to be discovered, Starship 1 is now almost certainly "the earliest arcade game yet known that clearly meets the definition of an Easter egg."