Demo units of the Nintendo 3DS are out in many major retailers now. They autoload Pilotwings Resort, or whatever game they're demonstrating, so you can't access the device's menu. Well, now you can, so you can see for yourself what lies beneath.

The instruction, per a commenter at Nintendo 3DS Blog:

Hold the power button until the 3DS is off.
Press and hold simultaneously the following buttons for 3 seconds: start, select, L, and R.
Press the power button and a menu should pop up on screen that asks about a StreetPass feature.
Press the home button to go to the menu.

That video above comes from another of their readers, showing the hack in action.

How to Access a 3DS Demo Unit's Menu [Nintendo 3DS Blog]