In 1979, when the Walkman was still in its infancy, Sony intended to include two headphone jacks, for two reasons. The first might seem a little silly in hindsight, but they figured that people would feel like utter fools walking around in public with a pair of headphones on. Instead, they envisioned two people sitting together in relative privacy, listening to music together. This seems to me even more proposterous; why wear headphones, consume batteries, and use an expensive portable unit when you can use plain ol' speakers and a tape teck and still verbally communicate with each other?
The second reason they did this ties into the writeup above -- by including only one jack, they figured people would be perceived as rude for trying to immerse themselves and only themselves in their own choice of music with no regard for those around them (but consider the alternative; flashback to the typical 80's movie featuring "guy with boom box listening to loud music on bus" e.g. Star Trek IV). Since people usually don't like to be perceived as rude, they figured they would avoid going out in public with a single pair of headphones (although I kind of doubt that they seriously expected two people to walk in tandem listening to the same Walkman). The Walkman was originally conceived when Ibuka Masaru, Honorary Chairman of Sony at the time, mentioned that he would prefer not to have to lug around a tape deck in order to listen to music on airplanes. At the time, the concept of a tape player without recording capabilities was unheard of, which is why the idea of a portable tape player never occurred to anyone until then.
Sony quickly discovered that people didn't mind being perceived as rude, or at least didn't consider the act of wearing headphones in public rude, and went on a massive campaign to advertise their wares by sending employees off to ride trains, attend public events, and walk through the streets wearing Walkmans (Walkmen?). As an aside, wouldn't it be nice if most products today were advertised in such a way, so that you could see them in use without being bombarded with pointless imperative orders to purchase something you've never needed before? The rest, as they say, is history.
As someone who regularly has to take (dis)advantage of public transportation, I can vouch for the powerful social numbing effects of the Walkman. Even in the relatively tame city of downtown Denver ("tame" of course is relative; I know of gangsta wannabes in Golden who think people downtown will shoot you for a nickel), one will run into some seriously interesting / weird / scary people whilst embarking on a typical public transit journey. I've seen my share of raving lunatics, and approaching-belligerent drunks. The Walkman (actually, I prefer to use my relatively cheap and godly bad-ass Rio MP3-CD player with Sony headphones) is an effective way of saying "don't talk to me. I am listening to something else, which I have chosen in order to avoid having to listen to anything undesirable. Feel free to sit next to me if you wish, but do not expect me to listen to a single word you care to say. You may speak all you want, but it would be foolish for you to assume that I've heard a word you've said, or that I will respond in kind."
I can't say how many times I've been thankful for having it with me. I don't actually use it most of the time, but sometimes it's absolutely necessary. Consider the case of having the back of the bus full of rambunctious adolescents who don't seem to understand the concept of noise pollution. Being a relatively non-confrontational person, I can simply don my headphones and tune out the rest of the world. That of course depends on the type of music, which must be chosen carefully in these situations to effectively drown out the disturbance. All the while, I see every other headphone-free non-confrontational sucker periodically glance to the back of the us with the evil eye, as if to say "shut the fuck up you rude bastards" without having to say a thing.
In this day and age the question of rights and to what degree we are entitled to them is more hotly debatable than ever. Do we have a right to be free from persistent sonic annoyance while riding the bus? If not, at least we can resort to this passive, gentle way of saying "fuck that noise."
While it might seem rude to do this, I do try to be as personable as possible with people I don't have good reason to be frightened of, and it really sickens me when I try to be friendly with people by simply saying "hi" at a bus stop, only to have them sit there in total silence as if I hadn't said anything. Sometimes that almost brings out the insane person within me, as I just want to say "Hello?!? DID YOU HEAR ME? I SAID HI!" I have encountered at least five people in only a few months who are either truly insane or simply want people to think so, and having headphones on is so commonplace that they never construe it as some attempt to shut them out. There's no sense in trying to figure out which it is, insane or just plain weird-ass and in need of attention...
And another thing! Nothing annoys me more than when I'm waiting at the bus stop blasting music at full volume and someone starts begging me for a cigarette. A)I can't hear you, B)I'm not smoking, why would you assume I have a cigarette? Rather than make the universal gesture of taking a drag off an imaginary cigarette so that I can simply make the universal gesture of shaking my head "no," they will often badger me verbally until I take off my headphones to hear the inevitable and respond with "I don't have any."