Given that the search for a new service sidearm for the US armed forces seems to have been resumed with the designation "Modular Handgun System", so should the discussion that somewhat subsided since the cancellation of the JCP in 2006 (or 2007?)
Reckon there are a lot of threads concerning that question, but in my opinion, the first step should be opting for a caliber and then choosing the weapon as a second step. If I'm right, other than the late JCP program, the MHS bidding (or pre-bidding) does not specify any actual caliber. Here are some thoughts about possible options:
5,7 x 28mm: From all I have read about that ammo, it would be the ideal military choice - high cap, low recoil, good penetration and still stopping power due to projectile tumbling in terminal ballistics. On the other hand, a lot of people consider that load rather a pimped .22lr... And, the use of subsonic versions with suppressed weapons seems useless since they a c t u a l l y don't have much more punch than a .22 in terms muzzle energy.
9 x 19mm: Still, the good old Luger seems to have its advantages: Apart from the fact that it's NATO standard (given the state of Europe's defence structures, I'm not sure whether the US should care too much about that issue) and the ammo is pretty cheap, there are also a lot of newly developed loads (including +P+, of course) with improved stopping power and even AP capacities (http://www.youtube.com/watch?v=ZyKMz3tYx-M
). Plus: Subsonic loads bear nearly the same energy as standard ones. I daresay that 9mm is one of the most versatile calibers (alongside 22lr...) and far from being outdated...
.40 S&W: Though I have never actually tried it, I guess we can agree that the 9mm is easier to handle in terms of recoil. The stopping power the FBI and a lot of federal agencies rely on seems to strongly depend on the use of expanding bullets which are not an option for a future military sidearm. Furthermore - the actual operators here on this forum may correct me - I reckon a military gun should allow the operator to get a lot of lead towards the enemy in a short time to buy time for a retreat or get the main weapon working again, not to achieve one-hit-Kills. Besides, the basic .40 is nearly as weak against body armor as the basic 9mm. Subsonic loads should be quite powerful, though.
finally: the good old .45 ACP. Most threads here seem to assume that the race will be run within that caliber, which is not unlikely given the JCP's caliber bias. Apart from its sentimental values, I must admit that I can't see too much advantage in the .45: inferior mag cap, strong recoil, less AP capacities even than 9mm or .40 - so what's the deal? OK, the .45 is subsonic by default, but still louder if suppressed than a 9mm due to the larger bore...
Well, these are my discussion inputs as a complete layman (I didn't shoot anything else than 9mm and .22lr, and even that was long ago), so please correct me and don't crucify me (especially concerning my remarks on the .45 ;-) )