Here's a great app called 'Android Protector' that password protects apps on your device when sharing it with others:
http://lifehacker.com/5695860/androi...ndroid-devices
http://www.youtube.com/watch?v=pbkWY-LBm9g

It can easily be toggled via the notification bar. Quite a nifty interface, and one that I know I will be using for minimum protection.