You may of heard some misinterpreted information... the SOC (Special Operations Command) ordered about 650,000 .45 ACP pistols, however, the entire military has not. As for now, the bulk of the Army, Marines, Navy, Air Force, and Coast Guard are sticking to the 9x19mm.
However, this could be a sign. In the 80's when Beretta came to the US with the M9, the Navy gave them to a few SEAL Teams. After using them in combat, the SEALs highly regarded the new 9mm pistol so the US began phasing it in. Now, the SEALs are again being issued a new pistol, I believe it's the .45 ACP HK Mk. 23. They seem to be liking it which made SOCOM buy up those few hundred thousand .45s. It seems to me that the military is gearing up to move back... they certainly could use it since high capacity .45s are available and the extra stopping power would be beneficial.
I also believe that many USMC SOC MEU's and Delta already use .45 ACP even though it's against NATO. However, there is a great potential for change in PDW caliber. I just wouldn't get too comfortable with the idea just yet, on account that the military spending budget has been cut due to the economy and therefore, the military would be very unwilling to phase in a totally new firearm (EXTREMELY EXPENSIVE) as well as retrain all of our troops to become familiar with the new pistol (also very costly and time consuming).