simple problem with an easy fix?
Had the same problem for a day or two. Walking around while listening to music would make the voice command activate or send my music into fast-forward. I realized by pressing on the base of the headphone cord where it plugs into the iphone I could reproduce the problem/s. connections inside the headphone jack are getting shorted out. Probably lint or something from being in your pocket/purse/whatever. So when you plug in your headphones later (at the gym or wherever), the lint gets pressed between the headphone jack and your iphone. Certain movements will press the jack against the lint causing it to short, activating unwanted commands (i.e., voice command, fast-forward, etc.,).
I got some Q-tips and ethanol (200 proof, rubbing alcohol) twirled the qtips around inside the headphone jack a few times (just once or twice may not be enough) and let it air dry. Don't soak the q-tip too much. Just dip it and wring out excess ethanol with your fingers. Don't try to dry it with another q-tip, you may lodge some cotton from the dry q-tip into the jack again. NO MORE PROBLEMS =) hope this helps.