The latest info I've seen on the ammo/rifle combo for the XM-8 is that it WILL be using 5.56mm to begin with, but it will retain the ability to easily and quickly switch to the 6.8mm SPC round if the US military so decides. The 6.8mm is gaining a bit of momentum, so this is not as farfetched as once thought.
As far as the barrel length issue: the XM-8's barrel can be anywhere from 10 inches to 24 in its SAW version. The "Standard" length is 16 inches, not ten, and there is a 20-inch tube available.
As some have suggested, the biggest problem with this is that going to a new cartridge is way tougher than going to a new rifle (not that we don't need one desperately). With new rounds, you need new mags, belt links, pouches, etc. Oh, and you ALSO, in the age of NATO, convince your allies to adopt the new round/rifle as well, shouldering that financial burden. Sure, us and the Brits could pull it off, but not some other, poorer countries.
Bottom line is, it'll likely happen, but over a longer period of time than some might prefer. Look for SPECWAR types to be the first on the block with the new rifle/ammo combination, followed by the infantry and Marines with the 5.56mm version. That'll be replaced in good time, as more ammo and parts become available. Give a total swap about a decade and a half.
IMHO, the "Garand-and Thompson, along with BAR and 1919A6" buffett worked fine. Something similar would no doubt work well today (perhaps if they had used the FAL in .280 and the HK UMP-45 together?), but the preference at the top is for "all-purpose" multitask weapons.
I say lets arm the "worlds policemen" with a decent rifle/catridge combination and screw the French and Germans. The Brits are our most reliable allie so partnering with them would be fine. Their current rifle is a total disaster, the weird bullpup thing. So they need a new one too. Other than that, who cares.
Seems Ronnie Barrett is "first on the block" with a civilian version. Oh, and it's available as a conversion kit, too!