While flipping through the latest EGM mag I read the low-down on the GBA. They have a picture of the GBA and it explains all the parts of the GBA. There was a nifty easter egg listed for the On/Off Switch.

First, you will need a GBA and a GBA game. Put the game in and hold down both start and select and flick on the power. You'll hear a nifty sound effect from Super Mario World. The GBA will pause the load until you hit a button, then it'll play a little confirmation beep and continue to load the game. If you don't remember, the sound is the same one you hear when you exit a level in SMW by pressing start and select together, only it's a bit faster on the GBA. I know I've heard the "confirmation" sound before, but I can't remember were. Below, are the sound bites:

Normal GBA boot up sound (0:04; 29.4kb)

Alternate GBA boot up sound (0:03; 26.9kb)

Alternate GBA boot up sound with delay (0:06; 46.3kb)