The "new" iPad is the same height and length, but 0.6 mm thicker. Also a little heavier, and it's not sure yet whether there is any change to the amount of curve on the back. And while the pics look identical, it's possible there are slight changes in locations of the switches, mic & speakers, camera, etc. So a soft, flexible case (leather, etc) should be fine, but some of the hard form-fitting cases may not fit as well... and it's possible that some of the holes won't match.
My recommendation: buy a fairly sturdy but cheap iPad2 case, a soft one, and then keep an eye out for the manufacturers to develop cases that are custom for the newPad. Nobody has built one yet for the newPad, they're just relying on it being similar enough to the iPad2 to market them for both.