Ok, so about the only thing actually in writing I've found is from Michigan.
This is directly from their state gov website:
Question: I am a valid medical marihuana patient under another state's law. Am I protected?
Answer: Yes, under Section 4(j) of the MMMA, a registry identification card or its equivalent issued by another state government to permit the medical use of marihuana by a qualifying patient or to permit a person to assist with a qualify patient's medical use of marihuana has the same force and effect as a registry identification card issued by the Department.
Of course, since there are no dispensaries there, it doesn't do any good.