Answered on Mar 28th, 2011 at 9:07 AM
If what you are asking about is carrying NO form of certificate for use, then you are simply violating the law, no matter where you are. If what you are asking about is the common medical marijuana certificate that users go and buy for $40 or $50 from dispensaries and co-ops advertizing in the back of local magazines, then no. California is about the only Politically Correct state blatantly violating federal drug laws by allowing issuance of the delusional medical marijuana certificate. It certainly doesnt apply in any other state, and doesnt even apply in CA against federal drug charges.
Now, despite that, there are legitimate federally approved and state certified, doctor authorized medical use certificates. With one of those, he is reasonably safe from arrest, but not guaranteed outside the state of issuance.