Reading the paper this morning, I saw this article about an airborne laser "gun" designed to kill tanks from 10 miles (15kms) away. Pretty impressive considering that tank armour can be 300mm thick and beyond.
Whilst a great idea in theory, and obviously big-shiny technology always looks impressive (which is why billions are spent on high-tech planes rather than making them cheaper and in greater quantities), I do feel that this plane is going to be rather limited as it is totally reliant on air superiority.
Currently it is mounted on a converted 747, with plans to later fit in to the venerable Hercules. Certainly on my travels, I have not considered the 747 to be the most agile of craft and while 10 miles sounds like a considerable distance, it is not that deep for a battlefield as they can stretch to several miles by the time reserves are taken into consideration. As such, its use is going to be limited to insurgent situations and the flight crew will have to hope like hell that they don't possess any cheap anti-aircraft missiles, which the US government would have in no way supplied to them. And if you believe what's in store for the future, it's not going to be of much use.
Even assuming that the future is not so bleak, the limits of the laser means that at maximum range the laser is going to be degraded substantially by atmospheric conditions (smoke and dust from the battlefield, water vapour in the atmosphere), which will causing "blooming" and reduce the effectiveness of the beam. Still, with a skilled controller they should be able to beam "USA! USA! USA!" into the clouds.