The passcode doesn't go on immediately unless you press the power button and 'secure device'. Doubletapping the power button or using the slider just shut off the display and the device will lock based on your setting in Settings->Device Lock. Once the passcode is on you'll have to enter it whether you hit power and slide to unlock or just use the slider button.

EDIT: Sorry misread your post. I've never had the slider button go off accidentally, it doesn't seem easy to do. Setting a passcode to go on immediately would keep apps from launching and stuff.