Looking for a cute and harmless April Fool's day prank for your iPhone. Consider iphoneception. It's a site that creates one of four special effects that mimic the iPhone's home screen, making users think that something deeply weird has happened to their unit.
Best of all, at least in terms of pranking, it can be installed and run on any unlocked iPhone (i.e. one that doesn't use a passcode). That's because it's just an interactive web page -- so you don't need an Apple ID to add it to a friend's unit.
You select the effect you wish to use (there are four, including explosions, gravity, kittens, and zombies) and create a homepage bookmark for it. Launch it from Springboard, press the lock button, and your prank is ready to go.
The video that follows shows the effect in action. Incidentally, it was recorded live from the iPhone using a beta version of Reflection, which will offer recording features when it's released.
Hat tip: Henry Kuo