Having worked in the aerial firefighting industry, I can agree with most of the analysis to date on the dangers associated with using rigid wing, transport type aircraft for fire fighting operations. The stresses associated with dropping water are not conducive to longevity in these aircraft.
Even before they were put in to use as fire bombing aircraft, the C-130A models provided to those companies by the government had a history of wing spar failures due to fatigue... that's the biggest reason they were removed from service. Supplying this aircraft to the fire bombing companies was a huge mistake by the government in the first place.... C-130s are not good fire bombers.
P-3's are also not a good platform for a fire bomber, as the wing is too rigid, and the "A" model P-3's being supplied were also known for having their wings fall off... the civialian version of the P-3 was the Lockheed Electra turbo-prop airliner, which again had a history for loosing their wings due to spar fatigue.
The P-2 Neptune on the other hand has one of the most flexible wings in service, and, other than a narrow body / limited wing spar / bomb bay clearance, which limites tank capacity (from 1800-2200 gallons, depending on who manufactured the tanks), are an excellent firefighting platform. This limit also means that the P-2 is carrying less than half of it's max rated payload with a full load of retardant. The few aerial firefighting P-2's I've heard of being lost were due to pilot error, I don't know of one that was lost due to fatigue (though it is possible.)
The Japanese modified their P-2 Anti-Submarine Warfare aircraft to use turboprops, so it would be fairly straightforward to replace the R-3350 radial piston engines currently on the P-2 if this became a concern.
Non-pressurized aircraft, like the P2, also have fewer problems with fatigue due to pressurization / depressurization cycles... which both the P3 and C-130 suffer from.
Grounding all 33 of the larger air tankers was a stupid, reactionary move... the P-2's should be returned to service ASAP, as they are likely some of the safest aircraft to use for aerial firefighting. Grounding the C-130s, P-3s and other, World War II era aircraft (Like the PB4Y-2 which was lost in Colorado, a Navy single tail variant of the B-24!) makes sense, until they can be properly inspected / documented for return to service or permanent removal.