Rabbits have long ears to help them loose heat as their body temperatures are usually quite high, around 40 degrees Celsius. This is done by circulating blood through a large vein that runs through their ears helping them cool their bodies.
1 Additional Answer
Rabbits have long ears because they are prey animals. Their long ears amplify sound so that they can run if a predator approaches. They are an early warning system. You can find more information here: http://www.4to40.com/Qa/print.asp?id=749