All that changed, of course, when the primary mission of the US Military evolved from war to occupation. American troops traveled around Iraq and Afghanistan in vehicles, often armored Bradleys and MRAPs, but mostly in Humvees, And when those vehicles were attacked, the soldiers found they had to get rounds downrange from inside a crowded vehicle. So instead of carrying a pistol an entire tour of duty without ever firing it in combat, soldiers would take their M9 and ten or twelve loaded magazines and push a real volume of fire downrange.
So it's perfectly reasonable that the M9 9mm pistols in inventory are at the end of their service life. It was adopted in the mid-eighties, and has seen action in three wars and uncounted deployments. But the Army is also concerned about the generally perceived 'lack of lethality' of the venerable 9 x 19 cartridge. This is actually hilarious, because the argument over the lethality and one-shot-stopping power of various popular handgun rounds has been the primary religious holy war in the handgun community for decades, the equivalent of Windows vs. Macintosh.
If not 9mm, then what? The obvious candidates are .40 S&W and .45 ACP. On the outside looking in are 10mm, .357 Sig and one of the modern PDW rounds like FN's 5.7mm. The FBI originally adopted the 10mm as their issue sidearm, but had to back away as they discovered that the weight, blast and recoil from this powerful weapon made training and effective use extremely difficult for many smaller-in-stature agents. It was their testing of some downloaded 10mm varieties that led to the development of .40 S&W. The .357 SIG is probably the ideal solution, but the high-velocity bottleneck cartridge design is probably just too 'exotic' for adoption by the Army. .45 ACP is probably not going to have the velocity and downrange accuracy to meet the Army's broad requirements.
Essentially, that leaves the .40 S&W as the only current off-the-shelf cartridge that would meet the demand for increased lethality. The problem that many law enforcement agencies that have adopted the wildly popular 'FortySmith' are encountering is that most of the handgun platforms they are using were designed for 9mm, and the additional pounding from the more powerful round has introduced reliability issues and shortened expected lifespans. So expect some specific language in the specs about strengthened frames, barrels and slides, along with design elements to reduce felt recoil.
So who's going to be the big winner? Glock and Heckler & Koch have very popular winning designs that can meet the anticipated requirements (with the exception of an old-fashioned demand for a 'positive safety'. Modern handguns tend to have no external safeties, so this demand would require a certain amount of re-tooling), but the Army took a lot of heat when it adopted the Beretta, a foreign product. Considering that in their recent (2010) competition, the ATF selected the Smith & Wesson M&P .40 over the Glock 17, I'd expect the Army to ultimately select that same handgun for their next generation service pistol.