Compared to a Predator UAV, the P-3 costs about five times as much, per hour, to operate. But the 60 ton P-3 can carry ten tons of weapons, along with the same weight in sensors, commo and computer gear. The navy won't say what mods are being made to support the anti-IED mission, but they probably have something to do with the radars and heat sensors already installed. These sensors have already been used successfully to detect people planting IEDs at night, and there has been a lot of research lately on modifying those sensors to detect the IEDs themselves. Whatever the new ideas are, there's plenty of room on the P-3, which normally carries 10-12 people to run the recon gear. The aircraft can stay in the air about twelve hours per sortie.
The U.S. Navy is getting some $210 million to modify some of its P-3 anti-submarine aircraft, and give them some capability to detect and destroy IEDs (Improvised Explosive Devices) in Iraq and Afghanistan. Several dozen P-3Cs have been flying ground recon missions since the 1990s (when P-3s were upgraded to perform search missions over land areas.) So far, about 65 P-3s can perform the land recon missions, and some of these are apparently getting the anti-IED upgrades. In this respect, the P-3s will be competing with UAVs like the Predator.