The iPad screen uses capacitive sensing technology. As such it does not respond to pressure. However, the contact surface of your finger increases as you exert more pressure. The extra surface area increases capacitance, and kind of acts like pressure. You can get the same affect by using the pad of your finger instead of the tip. There is a slight trade off in accuracy, but this should make little difference on a full sized iPad.
Cold hands (low circulation) are also less capacitive than warm hands. Worth holding in mind.
Hope that helps a bit.