The official response to the "Can I..." question is yes.
However, here are the issues. Using 5W charger on a device that requires a 10W charger, can and will damage the battery. It produces what is called a "trickle charge" which drains as it charges. Even when the iPad is turned off. This leads the smart sensors in the cells to go haywire and not know when it is fully charged or not. This often leads to people seeing their battery charge capacity lessen much faster over time.
The blocks used in the chargers are what regulate the wattage. A 5W charger cannot produce the needed 10W to properly charge the iPad, however, the iPd 10W charger, CAN and will regulate itself for the 5W device.
When combined with the proper components, this 10W charger usage on the iPhone, will not cause any damage or overheating in the phone, unless the device has already been damaged, typically via water/liquid. (proper components: Apple Charging block, Apple USB/Lightning cable, and undamaged dock/lightning port)
With the lightning cable and the iPhone 5/iPad 4 this will also charge the iPhone 5 faster than the 5W block. Added bonus.
EDIT: Sorry, the 10W is the "old" adapter (which BTW still works for the latest iPads) though they now deliver with a 12W adapter.