This may be my own opinion more than any cultural norm or hard rule or anything, but when you’re biking, I think it’s flat-out rude to blow by someone else without some form of acknowledgment. It doesn’t need to be huge–a simple “on your left” or ring of a bell would do, or a modest “hi.” Even a quick glance and smile, or a little waggle of your fingers as you go by would be nice. Anything to let the other person know that you’ve recognized them as another human being and are doing your best to coexist with them on this little stretch of road.
In a car, it’s easy to pretend like no one else exists except for you in your little mechanical bubble, but I find it inexcusable to act like that when there’s nothing at all separating you from the person two feet to your right who you’re about to go around. You don’t even have to be overly nice about it, but come on, at least let them know you’re passing.
Side note: whenever someone passes me with the straight-ahead 100-mile stare, I like to say “hello!” as cheerily as I can. It may be a little passive aggressively motivated, but it makes me feel better than just being annoyed at a rude passing job. And sometimes people will even look a little surprised and say hello back, as if they’ve been shocked back into realizing that I’m a person, too.
On my more grumpy days, however, I often fantasize about greeting the next person who passes me sans warning with a big old snot rocket to the face. So far, I haven’t;)