Unless you look at some sort of flip-type cover, your screen is going to be vulnerable. I'm not aware of any silicon or rubberized bumper-type cases that will protect the screen against actual throws.
Perhaps fixing the NOOK to a fixed surface is a solution? The screen might still be bashed, but at least it would require a concentrated (aimed) effort to do so. You could apply a screen protector to at least protect against abrasion, and put it behind a sealed facade to protect against spills. Make it more of a console/display than a portable (throwable) device. If you put some raised bumpers at the sides, it might even block attacks on the screen without blocking finger input.
Alternately, you could enclose the NOOK in a "chumby" style soft pillow case of some sort. If you're at all handy with sewing or crafts, this might be fairly easy to pull off (though charging might be a challenge). Embed a silicon or clip-style case in a soft exterior with the device recessed enough to be protected against most throws. The result might look disturbingly like a teletubbie, but perhaps that's not an altogether bad thing?