ive seen the 2hand mod, the sheild mod, but it would seem to work better to protect your backside, to help against backstabs, getting snuck up upon by creepers zombies, or a skeleton. simply a warning system, the wooden sheild could take 5 hits, the iron one 10 hits, and the diamond 20.
im sure weve all hit that situation in a cave where that one mob sneaks up behind us and backstabs us... could help alot for a quick defence. 5 hits on the wooden one because it is SO cheap. then it poofs into nothingness like all the other items!
it could look something like the capes notch gave us, but more of a sheild shape. probably could be tweaked with a texturepack too.
So... a piece of armor that is only useful while your back is turned. Let me think, so if I'm at full health and I get hit from behind I'm ok. So good so far. If I'm at low health and I see a creeper my best bet of survival is to turn my backside to it even if I can't run.
Don't see a problem, actually sounds like a surprisingly good and original idea.