The magnets on the smart cover aren't strong enough, with the feather case. It will stick, put the slightest bump or movement of the cover and the magnets give way.
I bought this same case, but in the color frost, rather than black shown here.
Incipio is designing a feather case to work with the smart cover. In fact they have pics of the new designed feather case on their webpage as of today.
Looks like the smart cover is applied to the iPad 2 then the new feather cover actually snaps over the magnet bar holding it all in place. The only thing "cut out" is for the 2 hinges on either side of the magnet bar.
This case looks good. They say it will start shipping beginning of May.