We're one day late on this one, but rather than brightly colored hidden eggs for Easter, in this case we're of course referring to "easter eggs," little secrets that programmers hide in their applications and code. Macworld has a great roundup of four easter eggs hidden in popular App Store apps, from a secret edition of the 1926 Kelly Blue Book in the official Kelly app to the funny result you get when searching for "tilt" in Mobile Safari.
Off the top of my head, the only other app easter egg I know of is the fact that you can play with the various characters wandering around the title screen of Halfbrick's Monster Dash. Yelp's Monocle AR service was originally an easter egg, although it is now part of the full app.
And it's not technically an easter egg, but some people may not know that you can access iTunes controls either by double clicking the home button while your iPhone is locked, or by double clicking it while unlocked to bring up the background app icons, and then swiping those to the right.
Any other hidden iOS features or app easter eggs that you know about? Share with the rest of us in the comments below!