For the 2nd and 3rd generation iPods, a remote control protocol was available through a non-industry-standard 4th ring on the headphone jack.
For the iPod Photo, iPod 5G, and most generations of the iPod Nano, the non-standard 4th ring on the headphone jack was re-purposed for use as a composite video output. (Maybe more models too? I don't have the specs.) This broke compatibility with anybody's existing previous-generation remotes. Newer remotes had to connect with these iPods through the dock connector.
With the iPhone, the 4th ring of the non-standard headphone jack was re-purposed once again, to act as a mic input. It also serves double-duty to act again as a rudimentary form of remote control input, but clearly not the same kind of remote control that existed in the 2nd and 3rd generation iPods.
With the 2nd generation iPod Touch, the 2nd generation iPod Classic, and the 4th generation iPod Nano, the non-stanard 4th ring is also used as a double-duty mic input and rudimentary remote control input. It appears that the remote control protocol has been augmented to include some extra actions that hadn't been possible (so far) on the iPhone.