Easter Egg discovery is a bit like genetic mutation. Get enough numbers to try some random stuff that's well outside of what they're supposed to be doing, let alone what they've evolved to do, and you'll hit something eventually. How apt that the Spore Creature Creator Demo, leaked yesterday and downloaded by lots of gamers, provides that lesson.
Reader Bahamut sends the Easter Egg, which is Spore creator Will Wright's head floating over the galaxy in the main menu screen. How'd he do it? By tinkering around. "Before I quit out of the entire game I try just randomly clicking around the main menu ..." Bahamut explains. Methinks he would have been among the first fishies crawling out of the ocean to breathe gas and not water. ("Before spawning and dying, I decided to randomly swim up this rock to see how far it was to the water's surface ...")