How well would glusterfs work across the Internet? Doing local HA with some kind of SAN seems fine, but the thing I'm trying to solve is "entire datacenter goes down, make the user have no more pain than making a new imap connection". It seems like something that would really need to be handled inside IMAP (using some crazy backing store for mail, like a database) to be done well, but maybe filesystem level replication with maildir would be enough.

I wish there were some "best practices in systems administration" for different services, ranging from simple (just set up imap/dovecot) through complex (gmail-scale), per service.

There's been pretty good focus on how to do http, dns, and some database services, but not as much recently on smtp, imap, etc.

glusterfs would be fine for your situation. There's just some tuning you'll want to do to handle the WAN situation without performance hits.