As EFF reports, large email providers (one assumes this includes GMail) have a practice of demanding warrants for email, even though the FBI is in some circumstances technically entitled to email with just a subpoena. The FBI apparently (as far as we know) hasn't litigated any of these cases, and has simply obtained warrants.

It doesn't seem unlikely that we're converging in the short term on a legal terrain where all access by law enforcement to email is going to require a warrant.