Well, a bit of curiosity and Google, have turned up this review article on the subject.
http://www.insuranceissues.org/PDFs/371W.pdf The case of medical marijuana raises some interesting questions. I expect that homeowners who lost medical marijuana could claim coverage of medication, as they could for any expensive prescription drugs in the house. The claim under damage to shrubs and trees cited in the article is truly (and ultimately unsuccessfully) imaginative.
The wrinkle with a designated grower under the medical marijuana law is that it is not clear who owns the pot from the point of view of the insurer. The designated grower only possesses the pot for the purpose of delivering it to the patient, and if the grower claims a personal ownership to it, he would likely run afoul of possession laws. The patient is not typically a guest in the grower's home, so her personal property would likely be excluded from coverage, or the grower might be found not to have an "insurable interest" in the stuff. The same issues might arise if you were storing stuff for an unrelated friend as a favor and a loss occurred.