You can read or download Dr. Marc B. Shapiro's article in the new issue of Milin Havivin here. The article is called "?האם יש חיוב להאמין שהזוהר נכתב על ידי רבי שמעון בן יוחאי"("Is There An Obligation To Believe that the Zohar Was Written By Rabbi Shimon ben Yochai?" ). While I thought the question was settled years ago (see here) and subsequently had to reconsider (here) you can read what Shapiro has to say in his interesting article. He brought some interesting sources to my attention, such as a Darke Moshe which says "שמעתי כי בעל ספר הזוהר הוא סתם רבי שמעון המוזכר בתלמוד שהוא רבי שמעון בן יוחאי," sounding a lot less committal than you'd think.
Once can also read some classic works on the question of the Zohar's authenticity:
R. Elijah Del Medigo - Bechinas Ha-das here
Yaavetz - Mitpachas Sefarim here
R. Moshe Kunitz - Ben Yochai here
Shir Rapoport - Nachalas Yehuda here
R. Leone Modena - Ari Nohem here
Shadal - Vikuach al Chochmas ha-Zohar, etc. link
R. Elia Benamozegh - Ta'am Le-shad here
R. Eliyahu Nissim - Ana Kesil here
R. Shlomo b"r Eliyahu Nissim - Aderes Eliyahu here
R. Moshe Yisrael Hazzan - Shearis Ha-nachalah here
R. David Luria - Kadmus Sefer Ha-zohar here
The above list is not meant to be exhaustive.
The entire issue of Milin Havivin 5 can be downloaded here.