The Live Earth event was intended to raise awareness about the environment. However, it also serves as a rather ironic event in many ways.
First, the event itself consumed a significant amount of resources and also generated pollution. People had to be transported to the events and a great deal of power was needed for the electronics involved (speakers, lights, transmitters and such). Of course, it can be argued that the positive effects offset these negative effects. If the awareness created by the events leads people to cut back on actions that harm the environment enough to at least offset the damage done by the events, then the event can be morally justified. Of course, there is a certain irony in having a massive energy-consuming event in order to fight environmental problems.
Second, it is rather ironic to have stars preaching about the environment. Many of the media stars involved in the event live in mansions, own numerous cars, and generally live excessive lifestyles. Some, such as Al Gore, even use private jets when they travel. Clearly, it is ironic for such people to participate in such an event.
Of course, the fact that many of those involved in the event life lives of excess does not disprove the message of the event. The truth of what someone says is not affected by their actions, however hypocritical they might be. For example, if a smoker says smoking is unhealthy, her claim is not shown to be false just because she smokes. To think otherwise is to commit the ad homimen tu quoque fallacy.
However, it is generally a good idea to make sure that those speaking out in favor of something actually live in a way consistent with that message. Perhaps the stars involved in Live Earth will be inspired by their own message and modify their lifestyles. Perhaps they will move to smaller houses, convert their mansions to something more environmentally friendly, and be content with one vehicle apiece.