Apple seems to have been the first to use a unibody metal design specifically for a laptop. Since Apple did it first, HP could be considered to have copied by some people.
Then again Porsche Design did the same thing for mobile phones years before Apple or any other consumer electronics company did - my personal opinion is that Porsche did most of the innovation in this area.
It's been a long time since I last saw one, I don't even know if they're still being made, but I recall the case being milled out of a chunk of magnesium alloy.
They're made out of magnesium and are multi-part. The trick is they make the internals somewhat waterproof (at least water passes through quickly) and suspend the entire mobo on rubber shocks.
Then again Porsche Design did the same thing for mobile phones years before Apple or any other consumer electronics company did - my personal opinion is that Porsche did most of the innovation in this area.