Won’t be anytime soon. Even if they started looking for a replacement today, they would not field a new rifle for years. The Army recently introduced a newer model of the M4 and they gave Colt a $9.5 million contract in February of this year for M4s and spare parts to run until September of 2012. The Army plans on using it for a while yet, maybe after 2012 they’ll get a new one. In the meantime any weapon that has been combat proven by our military for over 40 years is certainly reliable enough for any civilian application. Would a gas piston system be better? Maybe, but all of the good ones I have seen cost quite a bit more than a standard AR-15.
Which is the crux of the problem. We tend to buy weapons that are 80% of the best because they are 50% of the cost.
Heck, I'd prefer we let our soldiers take their own personal firearms with them to the front (provided they were of one of 2-3 standard calibers). Chances are you'd have better-maintained, higher reliability weapons best-suited for each soldier!