There is a feature present on current models of iPhone and iPod Touch that permits logging workout data with a little sensor made by Nike and sold separately. I picked up an extremely inexpensive sensor from eBay. I sure am glad I didn't spend a lot. It turns out that the battery is neither rechargeable nor replaceable. While there is a "sleep/wake" button on the device, there is no way to actually tell if your device is, in fact, asleep or awake.
The device, per the instruction manual, defaults to being on and active. Yes, let's use up that battery ASAP so the person has to replace their sensor repeatedly. Way to beat up on the environment, Nike. I hit the sleep button to put the device to sleep. I think. I can't tell if it's off or on, because instead of being a switch, it's an on/off toggle button and nothing changes visually to tell me what state the device is in.
Since all these design flaws translate into more frequent replacement of devices for Nike, I don't foresee their being rectified anytime soon. So I will experiment with the sensor I just got, but don't look for me to be replacing it when it's battery dies unless I find the cheapest possible deal. I am not going to be a lifetime subscriber to buying sensors.
Edit to add: HA! Two seconds of searching on the internet, and I have found battery replacement instructions for the sensor. Since the alternative is to discard the sensor and buy a new one, I wouldn't feel bad about attempting to open the device up once the battery dies. It's not like I'd be voiding a warranty or anything. So take THAT, Nike. I won't beat up on the environment and fill your pockets repeatedly.