Via Usability News, I found a press release saying 60-80% of British consumers have trouble reading their portable device screens outdoors, with 44% saying they were misled by advertising showing people using them in daylight.
Note that the press release quoted research sponsored by a next-gen screen vendor. While the data may be accurate, it's being released as blatant marketing spin to create a need for the vendor's products. Still, the data seems intuitive to me. My cell phone's large, bright screen is unreadable even in overcast conditions, let alone sunny skies.
Color-reflective displays, optimized for bright light viewing and low battery drain, have existed for years. Early implementations of the technology, however, caused a real backlash. First versions of the Game Boy Advance's dim screen, as well as the DS (pictured), were derided by industry press. These days manufacturers seem to rely on displays using the opposite technology, perhaps hoping consumers will grow inured to squinting and hand shading in order to use their cell phones and handhelds.
It doesn't have to be this way. Screens can be optimized for both types of ambient lighting. I always thought it was too expensive, but if a $100 laptop designed for children has a screen that works well both in daylight and indoors, why don't we all have one? So I hope pressure builds and consumers begin to reject their dim displays.