[thejeff]

Sometimes ignoring the message is the answer, sometimes its "I'm really busy and can barely play my turns and forgot about the message."
I generally try to make at least the first few messages in character, so I'll usually follow up an unanswered message with some kind of IC response to being ignored and a warning that I'll have no choice but to consider you hostile.

As for initial response, I'll sometimes stall by negotiating terms without actually agreeing to anything. Borders, length of treaty, even bring up cooperation against others.
Of course, I like to do some of this when I do want the treaty. Or I may be waiting for other negotiations. If I've got 2 potential targets, I may prefer to attack one, but if he offers a treaty, I don't want to reject it until I've got a deal with the other.