Call me a skeptic. But given Apple's well earned reputation for effective secrecy, how would the manufacturer have the external specs on iPad 3. The fact that Amazon says the case will fit the iPad 3 means little to me. All the rumors (and they are just that) suggest likely changes in form factor.
Probably the most significant of those RUMORS is a change in the camera opening. A small change in thickness of the iPad 3 might not make much difference in the case ( if it is not a tight fit). But if there is a change in the camera opening, or position, something that fits iPad 2 would not fit an iPad 3, unless the openings were huge.