I'm not a lawyer, but I remember being told during an introductory class to business law in school that the definition of what makes a trademark can be whatever is considered to be so tied to the brand identity that people would immediately assume that the product belongs to the brand.

In the case of the Macbooks vs Dell XPS ultrabooks you could argue that what makes people recognize a Macbook is as much the glowing white apple logo as the aluminum case so people are not being fooled when they see a Dell XPS into thinking it's a Macbook.

Similarly, people don't immediately assume that a sport car painted red is a Ferrari. But if they were to see a car with a Ferrari logo (regardless of its color) they would most certainly assume that it's a Ferrari.

In short, it's my understanding that trademarks are by definition somewhat subjective. However I believe that the intention is a good one, as it protects as much customers than brands.