You know, that whole acknowledges/doesn't acknowledge thing has been the topic of heated discussion on the local bike board. I agree with you, I'll say hi or wave or nod-it seems friendly and reasonable. I'm no great convo starter but I'll respond in some non-witty fashion if someone talks to me.
Being on a bike makes me feel a certain amount of camaraderie for others that share my need to pedal.

Some people don't care to communicate, I can respect that too (but I smell my armpits and wonder why )