Is there an Xoom Android tablet app for turning the screen on and off?
Yes. Screen Off and Lock is a Android app that let you turn off screen and lock device without touching power button. This tools allows you to switch off and lock screen by 1 click with screen off animation and sound effects. But it isn't a widget. I am unsure how to get back to the options page, but at least it does what i want it to.
Download Screen Off and Lock from android market.