My biggest worry, from seeing the demo is that the magnets, due to them having to not affect the compass, are not going to be very strong.
I feel the people running demo's had obviously had some practice in using them in such a way to look good.
I honestly can see them being a pain it the butt.
For an iPad sitting on your desk, I'm sure they will be great. For actually picking the iPad up, putting in in your bag, and other uses I think the smart cover could well be a pain as it will keep pulling away from the body.
You really want the lovel strong neodymium magnets that would take a good pull to remove the cover, and it would stay in place unless you really wanted to remove it. then of course there would of been compass problems.
I honestly can see these being annoying as you will go to so something and the cover will pull away.
Call me stupid, but I'd actually of preferred the think Steve joked about as being stupid. Some kind of mini stainless steel / titanium tiny thumbscrew to hold it on. Anything which meant I could actually hold the iPad up by the cover.
As has been said, without question these WILL scratch the iPad side over time, they will slide a little as they are pulled and sliding two soft metals will create abrasion marks over time.
As you suggested a clear plastic shield you be the answer, but that will, by a tiny amount reduce the magnetic strength a tiny bit more.
The concept is good. I just think it's got issues right now.