A security hole in iPhone 4 software allows you to make a call after dialing a few pound signs and timing a few others as found by a MacForums member.
Here’s a dude in Brazil showing it off.
More fun with the number pad from comments:
Open the number pad and type \”77\” for example and press dial. The screen dims, a loader comes up and after a while a message says \”Unknown Application\” with an option to dismiss the message.
Try it out!