posting replies by simply replying to mail from LJ:
To implement this, mail sent from LJ should be have Replyto: lj-<commentid>@livejournal.com; when user replies to , postfix grabs such emails and passes them to script, which will add comment.
Problem: authentification, solution: using one-time authentification codes, eg, Replyto: lj-<commentid>-<authcodeid>@livejornal.c
NB, this emails should contain References: field for email clients, which support threading.
posting journal entries: user sends email to some defined address and specifies username/password in some defined format. don't tell me about security, because in current http auth everything goes in plain text too.
reading friends: simple, get friends posts by email.
see how much I care about uucp users?