yeah, easter eggs have cropped up all over the place.. most of the time i'd be willing to bet that it's the developers' idea of a joke.. Anyone ever found the easter egg in one of the maps on Duke Nukem 3d, by turning on the Noclip cheat? (I dont remember where exactly, but if you use noclip and fly over to a room that has one of those green shrink-ray launchers in it, it seems to be an empty room -- until you turn on night-vision glasses! Then a message on the wall lights up... "YOU SHOULDN'T BE HERE." )

Personally, I never did know about the "Adventure" easter egg, while i was always a fan of that game.. Maybe I should blow the dust off my 2600 and go find it.. (or just download the rom.) I always associated the term 'easter egg' with finding a hidden object (message, room, etc) in a game.

(side note; My personal favorite easter egg (on the humor factor) has got to be the Quake 2 endgame "cyborg orgy"... I find that one to be absolutely hilarious)