From what I read from Handguns magazine (quoted below), the +P designation could also be read as "new modern safe threshold", and defines a new safe pressure limit for modern handguns. Warranties are sometimes based on SAAMI specs. Smith&Wesson has stated their guns will shoot any ammo within SAAMI specs. For modern pisltols, this should include ammo designated +P.

Well, is there a table or other reference that might help people know if a given ammo, say .44Mag, 300gr, 1100fps for instance - was going to be within SAMMI specs? Is the topic more complicated than that?

Here's the excerpt about +P loads from

Demystifying +P: The inside scoop on +P
By Dan Johnson

Most shooters know the "P" in the +P designation on a cartridge headstamp stands for pressure and indicates that the cartridge is loaded to higher chamber pressures and thus higher velocities. But many are confused as to exactly how much pressure is added and how safe these high-performance loads are. I believe this confusion is contributed to by people in the industry, some by firearms companies that understandably wish to err on the side of caution in our litigious society and some by small ammunition manufacturers looking for an edge in a highly competitive market.

Plus-P loads can, in some cases, boost the velocity of short-barreled .38 handguns enough to ensure reliable expansion. However, they do result in added stress on a firearm.

The +P designation came about for a very simple reason. As advancements were made in the quality and strength of both firearms and cartridge cases it was determined that some of the older rounds were capable of operating safely at higher chamber pressures in modern firearms than those originally established. Since firearms--and cases, for that matter--are durable goods that last for decades, even centuries, it was not feasible to simply increase the standard pressure specifications for these cartridges. There are too many old firearms around that could not handle the increase safely. So SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) uses the +P designation to separate the new pressure limit for these old cartridges from the old lower standard.

It is important to understand that SAAMI is the principle organization in the United States actively engaged in the development and promulgation of product standards for firearms and ammunition. Ammo specifications are not overseen by the Consumer Product Safety Commission or any other branch of government. Consumers should be aware that only manufacturers that are members of SAAMI are bound by the Institute's guidelines.