Google revolutionized the chat business - or at least the chat social network. With Yahoo and AIM I consciously added everyone to my chat list. Google skipped that step and put everyone I've ever communicated with in there. Generally its a good thing, but it gives people access to me who I haven't spent enough time with to share the same vision of how online communication works.
I find that when people have something important to say, they often start with extra hi.. how are you.. style smalltalk. Unfortunately this is the exact chat signature of people who simply want to pass time chatting.
There's no insult in just walking away from the computer, or not being at a computer at all. The green dot doesn't mean anything other than that it is possible to attempt to chat at this moment. The 'real world' takes precedent. I'm not going to ignore someone in real life.
I don't use chat as a way of getting to know people in the same small talk kind of way I would if I randomly met someone at a conference or a party. Unless we're really close either come to the point quickly in chat or send me email.
There's a certain protocol many people use in chat where there's lots of back and forth pings and acks before any real talking happens. Sometimes it feels like spoofing SMTP for its predictability. For me a simple 'hey' is good enough to determine presence, and then we can immediately talk about whatever.