Noted mythicist, Richard Carrier, has published a peer-reviewed paper that argues:
Analysis of the evidence from the works of Origen, Eusebius, and Hegesippus concludes that the reference to "Christ" in Josephus, Jewish Antiquities 20.200 is probably an accidental interpolation or scribal emendation and that the passage was never originally about Christ or Christians. It referred not to James the brother of Jesus Christ, but probably to James the brother of the Jewish high priest Jesus ben Damneus.
I'll be interested in seeing what if any reaction New Testament historians have. My guess is that it will be debated, as most things are among scholars. But I don't think it will have much impact on whether they think it reduces the probability that Jesus existed. For example, Bart Ehrman's summary of the evidence for Jesus's existence didn't rely upon Josephus's mention of him.
I would guess that in order for mythicists to be taken seriously, they will need to propose what is considered a legitimate theory that explains how it came about that people believed that Jesus actually existed.
HT: Adam Taylor