You can see Bart Ehrman's appearance on The Colbert Report (with thanks to Jim West on Xtalk for the heads-up) on-line:
Comedy Central: The Colbert Report
As I commented on Xtalk, it doesn't make easy watching. I'd say Colbert's tactics back-fired rather. If he is attempting to make fun of fundamentalist Christians, it certainly doesn't work. If he is attempting to make fun of Ehrman, then it just comes across as bullying. Just stop going on and actually listen to what your guest is saying! I must admit that I'd take Jon Stewart over the irritating Colbert every day of the week. Stewart's interview with Ehrman a few weeks ago was a model of how to engage intelligently, by contrast. (See Bart Ehrman on the Daily Show). For one thing, it was pretty clear that Stewart had actually read and enjoyed the book.
I once did a live programme in the UK where I felt that we were on more to be laughed at than to be interviewed, and watching Bart on this programme reminded me of that. It was a programme called The Big Breakfast on Channel 4, back in the spring of 2001, when I'd been involved with the BBC's Son of God (Jesus: The Complete Story in the USA). I was on with Jean Claude Bragard, who had directed that series. My policy was just to laugh along with them rather than to get antagonised, and Jean Claude was very good and just cracked jokes, which was about the right method, I thought. Not sure how I would have coped if I'd been on my own, though.
Update (23.50): thanks for the helpful comments (see below) and emails. I must admit that I've never been a big fan of the Colbert Report -- I have found it so loud and irritating that I've never really managed to get into it in the same way that I've (just about) managed with The Daily Show. It may be that it will take a little more time. At the moment, I watch it until about 11.40, having left the channel on after Jon Stewart, and then I get annoyed and switch it off. The comments are, of course, quite right -- Colbert is ironically playing a character who is parodying right wing, evangelical Christians. What I suppose I meant to say is that in this context I don't know how well it works given that Bart Ehrman did not appear to be particularly comfortable with Colbert's character's interview and thus any attempted attack on fundamentalism backfires. All I can say is that if it had been me, I would not have been too chuffed. But then Bart at least gets on these programmes. As I mentioned previously, Bart is now -- surely -- the new Dom Crossan.