Most of the posts about actual problems have been at cold temperatures, and I have seen reports of devices working ok at high temperatures (although you should test the screen in direct sunlight as soon as you get it, some screens fade and this is a manufacturing defect).
See
A Question of Temperatures! for a comprehensive answer from Laurent Picard of Bookeen. Note that the upper limit is due to the battery when charging (so don't charge it outside), and the screen is ok up to 50 °C.