Even when I'm green I'm not really here
My G1 keeps me always signed in to Gmail - e-mail and chat. So all during the day I appear as a green or orange ball indicating recency of activity. If someone sends me an IM my phone receives it and an LED starts to blink. Only, that LED is on my phone in my pocket and I'll look at my phone only occasionally during the day. So it's often hours before I see that I had an "instant" message. And, especially during the week, I'm usually busy with something and won't have time to reply, not even from the G1. So IMs go unanswered for hours or days at a time.
I'm not intentionally ignoring you; that's just being facilitated by technology.