This is in fact a really good idea
I actually think this is pretty close to being practical. You need an extra piece to give the iPad some vertical support, but that piece would be small, and you could leave it stuck to the refrigerator all the time.
The smart cover should do a pretty good job of keeping the top edge of the iPad snug against the refrigerator. And if the support below has a small lip, the iPad shouldn't slide out there, either. The smart cover is also strong enough to keep the iPad from sliding horizontally.
Here's an illustration of what I had in mind. The smart cover isn't shown, but it would be there as illustrated in the OP.
The support bar would have its own magnets (such as
these) to make it stick to the fridge. (These magnets are VERY strong - five of them should be more than enough to hold up an iPad.)
You can see a more detailed plan
here. It's really just a stick of wood with one rabbet and some holes drilled in it for the magnets.
Months from now, when I finally get my hands on an iPad 2, I think I'll build one of these...
