Design patent law is an area of great frustration for people. Design patents are relatively easy to obtain because of what they cover: essentially the identical design or any colorable imitation. As recently stated by the Fed. Cir., the test for design patent infringement is stated: "infringement will not be found unless the accused article âoeembod[ies] the patented design or any colorable imitation thereof.â" Egyptian Goddess, Inc. v. Swisa, Inc. Therefore, to invalidate, the design must either embody the prior art or merely be a colorable imitation. This is a tricky analysis.
For companies like Apple and Google, design patents are helpful in preventing knock-offs. In this respect, the line is blurred between trademark and design patent law. However, they are not useful for much else since many of the elements of a design are functional (and a good lawyer can make that argument) and are not merely composed of distinctive elements.
And, all is not lost for similar "prior art" designs, though. The very same case made the point that "prior art" designs might also be used by a defendant to highlight the differences between the claimed and accused design. Thus, an accused defendant might escape infringement by pointing out those elements they share with the prior art design and thus those elements cannot be the grounds for infringement.
A final point, design patents are what a lot of people were duped into filing when going to Invention Help companies. Those companies simply filed a mostly worthless design patent instead of a utility patent. They have practically no commercial value except as a deterrent to would-be second-comers trying to copy verbatim the design. Therefore, those that were duped have virtually no protection against second comers that merely make changes the look & feel of an "invention." Plus, the inventors are then locked out of filing a utility application because they usually don't realize until much too late (more than a year after they start selling, for instance). That sucks.