Running into a coworker/acquaintance/mortal enemy in the hallway can be one of life's most awkward situations. Print out College Humor's handy chart and keep it in your pocket, so you'll always be able to pretend you were reading and didn't see the person in the first place. (Click to enlarge.)
Where does everyone stand on that polite half-smile when you pass someone in a store or parking lot or wherever? That's just what people *do* around here, and deliberately avoiding acknowledgement feels like a snub. I'm trying hard to get past feeling like that, since I know many people are uncomfortable making eye contact with strangers.
That said, if a coworker refuses to look at me when I pass her/him in the hallway... yeah, I'm going to be a bit put-off. I'm old-fashioned that way. (Even though I know I shouldn't be.)