Here in Hell (Texas) where the summertime temps are routinely over 95 F (35 C) during the afternoon, I've had no problems with my Kindles while reading outside. I don't actually get into the direct sunlight though as 95 F plus direct sunlight equals suicide by Sun.
A few years ago the Sony PRS-300 I had would start to slow down as far as page turn speed went as it got hot. It would also start to get a lighter shade of gray for text and even show ghosting. As soon as it cooled down it returned to normal. But I've never had that problem with any of my ereaders since then.