Something like skynet is as yet not possible with current technology. In order for something of that magnitude to happen, you would need a computer that was sentient or powerful enough to compute enough lines of logic as to appear sentient. In that line, such a computer would still require external imput to manage itself and its goals.
Another problem arises with how such a computer could take over. If such an AI were created and did want to crush humanity, it would need access to weapons. In the event that it was capable of gaining control over a military force, someone could simply cut the power cable or the cable that the control came through to restore the system to their control.
It does make for great sci-fi material however. One of my notable favorites will always be
CABAL or Computer Aided Biological Assisted Lifeform. In his case however, he was part organic computer, and already had a massive military power under his direct control (as well as control over all of the military communication centers).
However, outside of some substantial jumps in computing and a sudden desire for everyone to hand over military control to it, I doubt it'll happen.