It hasn't happened yet, but Oliver Pike, the scientist who proposed the experiment, hopes that it will happen by 2015.
Although it's never been done in the lab before, the process is thought to be fairly straightforward. Scientists are going to smash together photons much like they do with particles in machines such as the Large Hadron Collider at CERN.
The only hard part about this should be finding the right place to do it. The experiment requires two powerful lasers and vacuum conditions, but Pike says several scientists are interested in trying it out, so it shouldn't take too long to find a place.
The experiment design relies on a hohlraum, a small metal cylinder that holds hydrogen fuel in laser fusion experiments. Heating a hohlraum with a laser produces a dense field of X-ray photons inside. Pike's team estimates that passing a jet of high-energy gamma-ray photons through the laser-excited hohlraum would make enough photons collide to produce up to 100,000 pairs of electrons and positrons.