The iPad 3/4 is the worst for heat as the backlight generates so much heat internally.
The best way to keep an iPad cool is;
1. Keep it out of the direct sun if possible
2. Keep it out of a case - these act like little blankets to keep the warmth in. iPad's are passively cooled out of the back shell
3. Take it off charge.
4. Direct some airflow over it
The new iPad Air runs really cool - it gets as warm as the iPad 2 (and runs so much faster than the 3 and 4)
Bevan..