I really don't think it's intrinsically offensive to the listener to call ID proponents liars. Consider the following 3 statements:
1. I have a grandmother in Liverpool who is blind.
2. I have an aunt in London who has cancer.
3. I have a cousin in Bristol who has eczema.
One of those statements is false. How are you supposed to know which one?
When an ID proponent calling himself Dr Hovind tells you that there are no transitional fossils and an evolutionist calling himself Dr Miller states that there are, there's no way around the fact that one of them isn't right. EITHER Dr Miller is lying about the existence of fossils
or "Dr" Hovind is lying about the non-existence of fossils
or somehow, despite arguing with evolutionists for a living, no-one has ever told "Dr" Hovind of the existence of transitional fossils. (I suppose, if you really, really wanted to, you could claim that paelentologists have been collectively lying to Dr Miller, along with everyone else, but the nature of his claim is such that you can't say he's ignorant of the non-existence of such fossils). I'd suggest that it's really much less offensive for someone to be the victim of a deception than to have bought into a dumb argument.
It is important that people understand that it has no scientific basis whatsoever. I couldn't agree more that it's important to explain
why. The only reason I mentioned the personal disingenuity of some of the more prominent IDers was because you said you figured an out-and-out creationist would identify themselves as such, and I lamented that we weren't dealing with honest people. Ideally, of course it's best to make the arguments that the evolution of the bacterial flagellar motor has arisen from other much simpler systems that have been cobbled together from doing other jobs; that the "irreducibly complex" blood clotting system in humans has been found with one fewer component in dolphins and wales, and with four fewer components in puffer fish, and that Darwin predicted the existence of an intermediate between birds and dinosaurs before Archaeopterix was discovered, and that palaentolgists know perfectly well that C-14 dating isn't a reliable way to assess the age of anything more than a few thousand years old, and that you can see evolution happening in systems like bacteria today, and that... etc.
However, when people are being told things by different people that directly contradict each other, I think it's necessary to explain honestly why the conflict arises. Theoretically anyone can go and find the evidence for all of this stuff, but in practice I'd guess that I wouldn't be able to read a professional biologist's paper on the topic, and I'll never have access to a lab or an archaeological dig. People who want to accquire a broad picture of how the world works sooner or later have to put their trust in someone; it takes at least 6 years from beginning university to getting a PhD in the UK, and usually more like 8. You simply can't know it all.
I agree entirely that you shouldn't insult the listener's intelligence, and that it's easy to come across as negative, arrogant or vitriolic when you're speaking about a proposal that is essentially devoid of all merit, apart perhaps from by some aesthetic criteria. I'm also fully aware that sentences like that one are derisive, but I don't think it's an overstatement, and the truthfulness of ID is not something that we're discussing here, so I don't see any harm in it.
To my mind, the absolute textbook demonstration of how to treat the topic can be found
here. It's the lecture by Ken Miller that I've already alluded to repeatedly in this thread. The lecture is an hour long with another hour of Q and A, but if you've got the time it's fantastic stuff. He is himself religious, so he's not an atheistic tub-thumper. He has an excellent sense of humour and a great way of communicating the evidence; he doesn't come across as arrogant or condescending. But he's absolutely unequivocal about the strength of the case for evolution, about the weakness of ID, and about the disingenuity of the proponents of ID.