If I made it a point to walk up to a specific person, look them in the eye, and greet them verbally, I would expect some kind of acknowledgement, preferably verbal and/or by gesture. As Amos pointed out, it's when someone just doesn't respond at all that is rude.

Perhaps present company excepted, it seems that men in general do more of a non-verbal, or monosyllabic greeting which leaves one to wonder if they've been heard at all. Maybe this is just a phenomenon of couples and they're just doing the Tim Allen "yes, dear" grunt to the "Honey dews.":-)

kat