Well, you may have a point about winter as I live up north in a colder environment. The air is dry in the winter allowing for charges to build and not dishcarge so easily but for us\electronics, this is not good. When there is a better amount of humidity, this aids in discharging those little electron buggers before we do. So cold, dry air will definately aid in a zap. We just bought my son an ipod this X-mas, and so far no zapping and he has worked out, etc...with them on but I will ask him regularly. I would think the headphones should NOT be a discharge agent unless very poorly made. I have had the old cassette, cd player, now MP3 players, with headphones, no zap. I have worked with many electronics and in my opinion, the ipod has an issue with not the headphones specifically, but grounding in general. I don't think we are shocking the headphones, this is a two way street here but poor equipment makes it more likely. The headphones are grounding the person to the ipod, or vice versa, causing this shock which means the ipod probably has it's own issues with static and is dishcarged through the easiest means as electricity takes the easiest path. The headphones are grounded to the ipod so in my opinion once again, it comes from the source, not the headphones. The touch pad may have something to do with this as you mentioned about grounding yourself out every so often. Either way, I think it's the ipod.
I listened to the ongoing debate about iPod headphone shockage with some interest, because I've collected quite a bit of personal experience with the iPod and getting the bejeezus zapped out of me.
I think there's definintely a weather component to the situation. I never got shocked by my iPod in San Francisco, but since moving to Boston it happens frequently, but only in the winter.
I also notice at the gym that while running no the treadmill I must be building up a lot of static charge (again only in winter). I touch those little heartrate sensor thingies every 10 seconds or so to get rid of the charge. SO I don't think this is really an iPod or headphone-specific thing, but if you're somehow accumulating static charge, and you don't touch anything else to get rid of it, eventually it finds a way from your body to the headphones. And that's when you feel it - sometimes quite strongly, if you've built up a lot of charge.
That's my theory: the headphones aren't shocking you. You're shocking the headphones!