It seems very rare now to hear anyone sing spontaneously in public (to themselves, not for an audience). If I do hear someone singing in public my first assumption is that they are drunk or mad. Is this just me? I suspect public singing got killed off by the jukebox and TV in bars and things went from there. Or perhaps things were always like this?
People (in the US, at least) seem much less likely to sing under practically any circumstances nowadays, except maybe in karaoke bars. ISTM that fewer people think of themselves as being “able to sing” at all; fewer schools offer art and music instruction, church congregations know fewer hymns, etc.
And of course recorded music is so ubiquitous, especially in portable forms like Walkmans and iPods, that just singing or humming a tune to yourself is pretty much obsolete.
Those of us who still do it try to keep it unobtrusive (singing loudly at a deserted bus stop is one thing, but on the bus or in a crowded elevator it would be rude and intrusive, unless you’re actually in a musical comedy or something). So maybe it’s a twofold effect: there are fewer people singing to themselves and they’re keeping quieter about it.
I sing, but as Kimstu said, I’m usually quiet about it… except in the car 
I think that there´s a slight taboo regarding singing, that if you don´t sing well you shouldn´t sing at all… although in the case of one of my coworkers that´s not the case since his screeches have the ability to tear eardrums.
Well, everyone on the street spontaneously breaking into song and choreographed dance numbers is a lot less common than it used to be.
Yeah, I miss those Dr. Pepper commercials, too.
A friend of mine hated musical theatre because he thought it was so unrealistic for people in the street to spontaniously break into song. That may be true today, but it wasn’t always so…
Shakespeare’s era in England is an example. Remember that everything about city life in England stunk. Literally. Death, disease, famine, livestock, open sewers, heads on pikes on London Bridge and The Thames River all contributed to a rather smelly place to be. Plus the water was never really safe to drink. So they did what civilizations have been doing for centuries to render their water potable - they turned it into booze. Substituting your drinking water for wine or ale was a great way to avoid water-borne ailments, plus being loaded most of the time rendered you pretty indifferent to the other unpleasantries of city life. It was not what we could call a sober city. And this is where the singing comes in…
Obviously, with the majority of the population on a decent buzz, city life was a lot more - energetic. Remember, the city was noisy enough as it was: Hooves on cobblestone streets, the calls of traders and chamberpots being dumped out of windows - it was a noisy place. And as people’s inhibitions decreased from the alcohol, the cacophony increased. Even normal conversation was loud, due to the tipsy state of the general public.
And singing in the streets? People sang readily, perhaps because of their malted euphoria. But also because the gap between music as pasttime and music as uplift did not exist as it does today. Singing was a way to come together and chase away the filth and stink that surrounded everyone.
My references for all this? Much of it comes from years of now-and-again research from my theatre work. But I do make specific references to an old photocopy I have in front of me. Sadly, the book’s title and author are not to be found - if I can come up with it, I’ll let you all know…
I think these days we’re used to musical accompaniment. People are more passive about music, it seems, also. A cappela is not terribly entertaining unless there’s a group. (I hate American Idol).
I agree that there’s a certain notion of not bothering people. Oddly enough, though, having my iPod increases the chances I’ll sing (and, btw, I am “able to sing”) to myself, but I keep an eye out for other pedestrians and dial it back when I’m coming near them.
People appear to have been far more ‘participiatory’ in music in decades past, when there was less (or no) electronic media distractions. I’d venture to guess that a smaller percentage of people on the whole play musical instruments as well.
As a record collector, some very interesting items that pop up are records that people either made at home on the holidays or had cut at a booth at carnivals. People just don’t seem to do that sort of thing any longer, unless they identify themselves as musicians.
One group I Americans I still see sing in public are rappers, with kids doing the street corner thing doo wop style to this day (we could debate whether or not this is ‘singing,’ but it is music and flows out of a street corner folk tradition.)
Construction workers still sing. With the help of the radio. Whenever there’s a popular [Dutch] song on, you’ll hear them sing loudly : “EEN BEETJE VERLIEFD, IK WAS EEN BEEEEETJE VERLIEFD”
I think it’s a nice change from whistling at your legs.
I love to sing myself. But I sing off key. And to show I’m not drunk or mad - like scm1001 so cleverly noticed - I just hum. Without moving my lips. 
I am exactly the same, though the bicycles that sneak up behind me fast often cop an earful of Dylan. But is this a new phenomenon, or did this idea of bothering people with ones singing always apply?