It has to got good battery life , viewable screen outside....
What you want is something like the Panasonic Toughbook. Most outdoor viewable laptops/tablets are built "tough" and as a result are pretty expensive compared to normal ones.
Its rumored (in a couple months) the next Apple MacBook Pros might be coming with a new Retina display which is what is used on the iPhone 4.
I've always wanted an outdoor viewable laptop so I look forward to what these new MacBook Pros might offer.