The Apple support article Keeping iPad within acceptable operating temperatures reminds users that it's never a good idea to leave the iPad in the car on a hot day. Duh!
The article suggests that the standard operating temperature range is 32º to 95º F (0º and 35º C) and working on the iPad when the temperature is above or below those conditions may "temporarily shorten battery life or cause the device to temporarily stop working properly."
Higher temperatures are more of a problem. Here is the list of issues you may experience if the device gets too warm:
•The device stops charging
•Weak cellular signal
•Temperature warning screen appears with the message "iPad needs to cool down before you can use it"
This message appears when the operating temperature has become too hot. This is a safety mechanism that protects the components of your device. If this message appears, you should turn the device off, move it to a cooler environment, and allow it to cool before resuming use.
Apple suggests that the following activities may contribute to a temperature warning message:
•Leaving the device in a car on a hot day.
•Leaving it in direct sunlight for extended amounts of time.
•Using certain applications in hot conditions or direct sunlight for long periods of time, such as GPS tracking in a car on a sunny day or listening to music while in direct sunlight.
So, maybe don't use the iPad long in direct sun on a hot day. Of course, the super-shiny glossy screen makes it difficult to use in those conditions anyway, so this could be a feature.