Plants vs. Zombies Easter Egg - He's Studying Your Plants.

Go to the achievements tap on the splash screen, scroll down to the end of the list, but notice you can keep scroll further down.
Then you will see another of Pop Caps characters: The Bookworm from the Bookwork series.

Scrool further down and you will see several gems (that you cant pick up), a zombie worm, skeletons and something that looks like a Mayan idol.

At the end you will reach China wich apparently is also filled with zombies.

Have fun... :)