Just stumbled across a number of very interesting works by an author I wasn't familiar with: Frederick R. Dannaway. I recall reading one of his articles (Thunder Among the Pines) a few years back, but until yesterday I hadn't encountered any of the others. It turns out that Dannaway has spent a lot of time examining the Soma question, and has uncovered some interesting references in that area. Some of the more interesting papers I found:

• Thunder Among the Pines: Defining a Pan-Asian Soma Explores soma as Amanita muscaria
  
• Plant as Guru and the Soma Admixture Theory Explores soma as Peganum harmala and the possibility of admixtures.
  
• Tea as Soma Explores tea as a sort of "soma subsitute" in Buddhism (proposing that rituals and associations that arose around soma transferred to tea)
  
• Celestial Botany: Entheogenic Traces in Islamic Mysticism Title says it all; references to and suggestions of psychoactive plants in Islamic traditions (again with some ties to soma).
  
• The Flower of Consciousness and The Lore of the Lotus: An Exploration of Lotus Symbolism in Sacred Art, Yoga and Ethnobotany A much-needed exploration of the role of the lotus in Eastern mysticism (again with some ties to soma).
  
• Bread of Heaven or Wines of Light: Entheogenic Legacies and Esoteric Cosmologies Covers "psychoactive preparations of ergot in various religious systems with a particular emphasis on Persian, Greek, Jewish and Islamic sources"
  
• A Toad on the Moon: Or a Brief Speculation on Chinese Psychoactive Toad Venoms A speculative look at Chinese toad imagery.
  

Yeah I have read his stuff. Pretty interesting.