I am having the same problem. I got this phone in January, and I have been running with it ever since. For several months, I actually had no problems with the phone or the headphones it came with. Then, in late April/early May, when the first hot and humid days came around, I had the voice control randomly activate and music fastforward/skip for the first time. For a few weeks, it would only happen on my very long runs and maybe only one out of every three runs. However, the problem has gotten much worse over time. Now, it typically occurs about a half hour into every run, and does not cease for more than 30-60 seconds at a time. It happens like this: it activates voice control, I attempt to click out of it, it starts fastforwarding the music, I try to pause the music and restart it, it still plays the music on fastforward, it may skip a few songs and then it settles on one song at a normal speed for about 30 seconds, it activates voice control again, and the cycle repeats.
This has rendered my iPhone essentially useless for running with music, because I am a half-marathon runner, so even my short workouts usually last more than 30 minutes. This is really a shame because my primary reason for purchasing an iPhone (and the majority of the use it gets) was dependent on the use of GPS-enabled apps for running. Additionally, over the course of this happening, the earbuds/headphones have lost some of their functionality... they still work to pause the music, but the microphone sometimes does not work, and the volume controls are completely nonfunctional.
It seems that some people are saying that this problem is occuring due to moisture entry into the earbuds through the microphone? The moisture/humidity part makes sense, given that I started having this problem when the weather got humid, and the fact that I never have this problem except when I am exercizing. However, even when I remove the earbuds from the phone, the problems with the phone (voice control activiation, music fastforwarding/skipping) continue for at least a few minutes, and I am forced to turn the phone off. The phone itself usually returns to normal working order after being inside an air conditioned environment for about 15 minutes.
It seems clear that the headphones are playing some role in this problem, but what I am wondering is if anyone knows if this is a problem that is related to the phone itself as well. Does the problem stop if you switch to new headphones, non-apple headphones, or headphones without microphones? I am curious if it's actually a problem isolated to the headphones, or if they phone itself is hypersensitive to humidity in the air. Or perhaps, the circuitry problems in the headphones somehow exacerbate the humidity sensitivity within the phone. I never had this problem with my 64G iPod Touch when I ran outside, but I also never had the apple headphones before.
Any help is appreciated. Good luck to everyone with this problem!