A design patent SHOULD be just part of trade dress. I think it's an American thing - the UK equivalent is "Registered Design".
And the UK legal standard is very clear - if an *informed* consumer would not be fooled, then there is no infringement. Basically, the likeness has to be so extreme as to seem intended to fool the consumer.
It makes sense - if a customer is tricked into buying the wrong product it's fraud. Which is why Judge Birss's comments in Apple vs Samsung were so pleasant to read - "Samsung's Galaxy isn't as cool ... Case Dismissed!!!"
Not that I agree with his idea of coolness, but it was very much "no observant customer would be fooled, so what the f... are you doing in my courtroom!".