DisplayMate just published the results of a test comparing tablets and smartphones under bright ambient lighting conditions. On the tablet side, the tests were run on an iPad 2, Xoom, Galaxy Tab 10.1, and Kindle Fire, with lighting that ranged from 0 lux (absolute darkness) to 40,000 lux (indirect sunlight, outdoors).
From our experience, most tablet use occurs indoors, and these test tablets all fared well through 300 lux, or moderate indoor lighting conditions. But once the light was amped up past 1,000 lux, test photos showed dramatic color degradation on both the Kindle Fire and Xoom. Colors looked very washed out and images lost the necessary contrast to discern edges--kind of important for reading text.