Well it depends on how you define mech. the CIA has successfully used drones in combat, and in the future we are probably going to see more and more autonomous drones on the battlefield . For those of you who have seen Minority report, imagine 2-300,000 "Spiders" crawling over the battlefield acting as one unit.
As for traditional mechs, its a little more problamatic. As has been mentioned, Gundam and Eva sized mech are pretty much impossible to build. If u double the size of an object, u quadruple its mass, so a mech the size of an eva would break its own legs.
Another problem with mechs is the fact that walking on two legs is very, very hard. The most advanced walking robots today can climb stares and thats about it. If we are going to see walking mechs in the armies of the future they would have to be able to outperform other forms of transportation, namely tracked and wheeled vehicled. Modern tanks can go 45mph, and many wheeled APCs can go over 70.
Now, in the future we may see small walking mechs that are an extension of ourselves, more of a powersuit, or an exoskeleton. In fact DARPA is working on
this project right now
DARPA is also working on other cool projects like
Brain Machine interface
and
morphing aircraft
-Metro