When you have your headphones on, frequently there is a current going through the headphones that produces a hiss, even when your not connected to a phone call or playing the iPod. I've yet to determine what the conditions are that cause a signal to be sent to the headphones. For example, prior to a 'custom ring' being played when you have an incoming call, the current will precede the sound as a quiet hiss. Even if you choose a built in ring tone, the sound (white noise/hiss) will precede the ring. If you leave your headphones on when your not using the phone, that hiss may start and stop at certain times.
What actually concerns me about this is that it is drawing power from the battery when I'm not using any audio function. I don't know if the speaker uses this as well if the headphones are not plugged in, since the gain is too low on the built-in speaker to hear it (without being in an anechoic chamber).
My iPhone 8Gb original (not the 3G) is using v2.1