Interesting article on this very subject here...
Tablet Displays Under Bright Ambient Lighting Shoot-Out
It would appear that at the moment the only half way decent Android tablet for your purposes is the Samsung Galaxy Tab 10.1, but even that suffers from considerable wash out under direct sunlight levels...
As the badly advised owner of a Nokia smartphone which is effectively only usable indoors, I found the article quite interesting. Given that the camera on the phone is quite capable of producing some good photographs, its a real shame you cant actually use it to its full potential outdoors because the display screen is totally washed out... For the same reason unless one places the device in a rectangular tube shaped box to shield the display from sunlight the GPS facility is a lost cause! Trying to read text or whatsapp messages outdoors is nigh on impossible...