Keeping clients honest

Here's a dilemma: what does one do with the client who can't keep track of its own deadlines?

In the past, when I've attempted to cover my behind by pointing out that the client hasn't provided the necessary materials in order for me to meet the original deadline, this information has been met with a revised timeline and occasionally a thank-you-for-pointing-that-out email. My most recent experience with this kind of communication was met with a terse I'm-too-busy-to-worry-about-deadlines response, implying that I was nuts to care about such a trivial thing as a project deadline.

That kind of tap dance really gets my dander up. When you give me a deadline, and I agree to it, until you tell me otherwise, I will stick to the original plan. My professionalism prevents me from letting deadlines slide, even if the client is the one doing the sliding.

Is it too much to ask for the client to just send a quick we're-moving-the-deadline email, with details to follow? For some clients, apparently it is.

(P.S. The obvious question here is why do I care more about my clients' deadlines than they do?)