On the earlier post, Baby's Blue Beads, someone left the following comment:
...the reaction to questioning the Zohar - either its authenticity or its authority - and delving into the superstitious elements or some outdated medieval concepts that underlie the work, will probably be the most visceral and I think more so than any subject, even the evolution issue. The difference is that you don't really see this happening in print, unlike the evolution issue which has been a fairly prominent issue, relatively speaking, in recent decades.
Personally, I think the reaction against an intellectually honest historical approach to the Zohar would be even stronger than the reaction against Rabbi Slifkin. Even many reasonable people who would not necessarily go "on the attack" over it would nonetheless feel it's out of bounds or offensive to question that work.
I think that he's probably correct. You don't mess with the Zohar. Even a relatively moderate charedi figure such as Rav Leff said that somebody who does not believe that the Zohar was written by Rav Shimon Bar Yochai is a heretic (see this link for audio, transcript and analysis). I am told that Rav Leff did later retract this upon being made aware of various information, but the fact that he said it at all shows how widespread such a view is.
It is fairly well known that Rav Yaakov Emden challenged the origins of much of the Zohar, but this doesn't seem to be taken very seriously in traditionalist circles today. Perhaps this is because Rav Emden's opposition to Rav Yonasan Eybeshitz as a Sabbatean, which is popularly (though probably mistakenly) considered unfounded, gives him an image of someone who unjustifiably opposes things. But in the course of my research for my Shiluach HaKein article, I was surprised to find that even a figure as conservative as Chassam Sofer was of the opinion that most of the Zohar was written in a much later period. One would think that the Chassam Sofer would be an unimpeachable authority, but it seems that his views on the Zohar are not widely known.
Personally I have never really explored the issue, beyond the aforementioned view of the Chassam Sofer. There was an article on this which was floating around the net a few years ago, which you can download at this link. I can't give it a haskamah, since I'm not nearly knowledgeable enough in this area to evaluate it, and I haven't even read it carefully; just enough to see that it needs quite a bit of editing! But the quotations at the end, from unnamed Charedi gedolim, are fascinating and show just how divisive and explosive this issue is.
UPDATE: See too this post: Rav Ovadiah Yosef on the Zohar