I'm pretty sure the first was the infamous "invisible dot" in Atari 2600 Adventure that led to the room that said "Created by Warren Robinette."

The origin of the term, hmm...I'm pretty sure somone at Atari's magazine christened it an easter egg when a kid wrote to them asking about the Adventure dot. Kind of makes sence, because looking for the dot was like going on an Easter egg hunt, right?

BTW, personally it annyos the HELL out of me that "easter eggs" are such a big deal in DVDs. If I'm paying my money for a DVD, I don't want to have to get on the internet to find the codes or whatever to access half the features on the disc. When it was just developers hiding their names, that's one thing now, but I'm waiting for the DVD where the entire movie is an easter egg.