Suspecting a problem with the "virtual" uids/gids, I've tried with a real user for the mail user:

# 2.1.14: /_ETC/dovecot/dovecot.conf
# OS: Darwin 12.2.1 x86_64  
auth_verbose = yes
disable_plaintext_auth = no
mail_debug = yes
mail_gid = dovemailer
mail_location = mbox:~/mboxes:INBOX=~/mboxes/inbox
mail_uid = dovemailer
passdb {
  args = /_ETC/dovecot/db.conf
  driver = sql
}
ssl = no
userdb {
  driver = prefetch
}

Of course, the db queries do not return uid/gid pairs anymore.

And the pop3 processes now seem to work as expected; connecting for the first time as user1:

$ telnet 127.0.0.1 110
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
+OK Dovecot ready.
user user1
+OK
pass pass1
+OK Logged in.
list
+OK 0 messages:
.
quit
+OK Logging out.
Connection closed by foreign host.

Hmmm...

Axel