The Uncredible Hallq: I wouldn't pay any attention to Ben Witherington
Having looked at the comment Hallq is referring to by Witherington I'd have to say, yes it commits the ad hominem fallacy against Bart Ehrman. This is inexcusable and regrettable. However, Hallq goes on to suggest that Witherington can't be taken seriously because he has, at least once in his life, committed the ad hominem fallacy in the combox of a blog (his own, to be sure). So Hallq's argument must have the form (in order to make it valid)
1. Witherington once committed the ad hominem fallacy.
2. No one who ever commits the ad hominem fallacy can be taken seriously with respect to anything he or she says.
3. Therefore, Witherington cannot be taken seriously.
I'm afraid this argument itself commits the ad hominem fallacy. For those of us who inhabit the blogosphere, I'm guessing it isn't as easy as it looks to find much of anyone who, at one time or another, who hasn't committed the ad hominem fallacy in the combox of a blog. It is especially interesting that he was tipped off to this by Steven Carr, who, I think, (correct me if I'm wrong) at least once in his life at least or more often, has committed the ad hominem fallacy in the combox of a blog.
Witherington has presented a substantive critique of Ehrman's work, but this, of course, can't be taken seriously, because he committed the ad hominem fallacy against him in the combox of his blog.
There's no substitute for evaluating serious arguments on their own merits, even if they come from people who commit the ad hominem fallacy. Discrediting people is easier than refuting arguments, but it has all the advantages of theft over honest toil.
I repeat, none of this excuses Witherington for committing the ad hominem fallacy.