I think that a better example is Coke -- their bottle shape is a user-friendly design that is easy to hold onto, but more importantly it has become an iconic symbol of Coke the brand.

I think in the case of Apple, they iterate so often on basic laptop design that having trademarked trade dress around a laptop would be counter-productive. They always want their devices to be thinner and lighter, and that requires fundamental changes to the design (wedge shapes, unibody, etc)

With the Dell laptop, what really makes it look like Apple? It's silver and thin. There are literally dozens of vendors shipping thin, silver devices.