There are about 18 different kinds of these things. There are the "instantly twisted, horribly uncomfortable, proprietary plug" headsets that can be used only with specific models. I always figured this was the manufacturer's way of telling its customers: "Do not listen to this device using headphones." (See Figure 3.)
Fig. 3: Proprietary plug headsets are more trouble than they're worth.
Then there's the 3.5mm (1/8-inch) "miniplug mono headsets," like those dorky Plantronics things telemarketers wear. I remember the first time I saw one of these: I looked at the plug, saw three conductors, and thought, "Uh, stereo?" No, that's mono plus mic, but it looks just like a stereo miniplug.
Fig. 4: The mono headset connector looks exactly like a stereo mini-plug, but don't try using it for music playback.
Ironically, some music phones are equipped with 2.5mm minijacks, so you have to use an adapter if you want to listen to them using regular headphones, like those in Figure 5.
Fig. 5: The standard Walkman-style, 3.5mm stereo miniphone plug is found on everything from iPod headphones to high-end studio cans, but implemented inconsistently on cell phones.
Further, the plug on the 2.5mm-to-3.5mm RadioShack adapter in Figure 6 looks exactly like a normal headphone plug — three conductors — but it doesn't work, because you need four conductors, for left, right, ground, and mic. The gray adapter in Figure 6, sold by T-Mobile, will route the audio correctly for a T-Mobile Sidekick, but it may not work on other cell phones, because there's no standard for this stuff.
Fig. 6: Adapters are ultimately pointless for mobile devices.
The iPhone, for example, uses a four-conductor jack that is compatible with standard, three-conductor headphone plugs (you use the mic on the iPhone itself), but the jack housing is so recessed into the case that standard plugs don't fit in without surgery.
There are adapters to solve that, too, but it doesn't really matter, because adapters are so confusing and aggravating for customers, nobody uses them anyway. Show me a phone without a standard headphone jack on it, and I'll show you a phone nobody's using for listening to music.