I think we need to make a choice as a society where the line is drawn. We chose to allow privilege for religious, medical, and legal conversations. Sure, in individual cases, it would be better to get the data from someone's confession, but overall, it's better for society (or people decided) to allow those conversations to encourage religious confession.
Maybe there needs to be special protection for certain classes of cloud service, computing service, or communication. Certainly allowing people to use an "exocortex" without fear of seizure would make people smarter. It might make some crimes harder to punish.
Luckily, technology gets a vote, too.
I think a clear/easy line is that anything which is "personal thought" or approximates thought should be immune to search, ideally though technical means. Notes (for yourself), a journal, etc. Maybe "quantified self" measurements. etc.
The line is probably in a different place than in the telephone era, or even the disconnected Internet era.
I'd prefer it be defined through legislation (and maybe through constitutional amendment) vs. through legal decisions. The problem with legal decisions is they tend to involve criminal cases, and "a person was keeping a personal diary of his child rapes" is an exceptionally hard thing to argue privacy for, even if that's only 0.001% of the use case enabled by making personal notes private.