"Design patents" are more sensibly understood as being part of the trademark regime than the patent regime. They protect entirely arbitrary and non-functional aspects of a product. If the shape has any utility, then the design patent is invalid.

I personally don't see the point of design patents. The whole premise underlying the patent system is to prevent free-riding, but shapes are rarely the product of significant capital investment. If the problem becomes one of confusing similarity, then trade dress is the more appropriate vehicle (incidentally, I think if design patents hadn't existed, Apple would've sued Samsung on trade dress infringement grounds).

There is an internal tension in IP law between the economic justifications for the law and, well, the law. The point is to protect expensive capital investment into design from free-riding, but the law is loathe to actually look at whether a design is the product of expensive investment.