I think one of the problems with talking to believers is that the belief is often (but not always) tied up in the self perception of the individual. Look at at the 'discourse' among scientist with mutally exclusive ideas: it gets pretty heated.
Challenging the belief is like challenging their personal qualities. In my job I can't mention religious beliefs at all! And my job is to look at peoples every day beliefs and perceptions with the intent to alleviate depression and anxiety.
It's very hard to change belief. The spectre of "Yes but......" hoves into view. Being English I'm temped to say "Let them beleive what they like" but when I see George W acting like a religious leader with nukes it puts the fear of god into me.
What can we do? Moderate our discourse, don't go off topic, stick to one concept in a tread, not have 3+ discussions going on. Spot the logical fallicies and point them out and above all don't go round in circles. If the message is not getting across don't waste your time.
Stick to science. I don't think that any creo can stand against the scientific approach. (ok, so thats just my belief)