Google's particular problem, in my humble opinion, is that the protections do not apply to non-content portions of the communication. For example, the government does not need a warrant to require production of the smtp logs, just as they don't need a warrant to see what phone numbers you called whereas listening to those same conversations does require court authority.
Google unlike most other email providers is going through the content of every email (I assume prior to its being read by the user) and indexing its contents for the purposes of determining relevant advertising (and whatever else they do with that info about which I admittedly know very little).
I would wager that those indexes might fall under the "log" rather than "content" aspect and therefore their production would not be subject to warrant if they are keeping it stored somewhere; but, that's for someone receiving a higher pay grade to determine.
Cert for Warshak was not sought by either party most likely because the outcome of the appeal was largely in the govt's favor. Most of the convictions stood while only some were remanded. So I imagine it wasn't in either parties' best interest to roll the dice again. Finally, the precedent established in Warshak is applicable in the other horizontal jurisdictions. I would find it hard to believe any of the other appellate courts would go against its sound reasoning. But I guess the point stands that if you want to be safe you should ensure your servers reside in the jurisdiction of the Sixth!