The first WELL-KNOWN easter egg was in Adventure, but that doesn't make it the first. It seems like Fairchild might have beat Warren Robinett to the punch, but I don't think Warren knew about it (or any others) when he put his name in the game.

Oh, and looking back at an old post, the "dot" was never referred to as an "easter egg" (as the name of the object) that I've ever seen -- it's always been referred to as "the dot", "the magic dot", or something similar.