Found this on a dedicated pac-man website, I don't know if it's true or not, but thought I'd pass it along.
Original Pac-Man Game Hidden In Galaga/Ms. Pac-Man Reunion Game!
If this is true it could be one of the coolest easter egg of all time.
- Insert coin(s) into machine
- At the "press start button" screen, at which you can choose either Ms. Pac-Man or Galaga, move the joystick in the following pattern: up, up, up, down, down, down, left, right, left, right, left. You'll hear a sound, and the Ms. Pac-Man character on the screen will change color. Then hit the Ms. Pac-Man start button and (drum roll...) YOU WILL BE PLAYING PAC-MAN.