I don't have a fancy phone, and I definitely don't have a dozen fancy phones, so I'm not the best person to guess how email is implemented on them.
I don't want to make people re-type a huge authentication blurb everytime in their body, so I'm thinking the auth should be in the email address, which presumably you can add to your address books?
The LJ password won't be part of this auth. If you want to post by phone, you'll have to setup a special PIN for it that's good for nothing but posting. (assume it'll be compromised somehow... people will look at your cellphone address book)
I'm thinking of form:
No security options, at least initially. Everything is public, unless you've set newpost_minsecurity.
What about subjects? Do phones support those? Mine only allows emailing as a type of text message, and I don't have subjects on text messages.
Do we let subjects be faked if there's no native support?
This leads to possibly doing auth in subject/body. Perhaps auth could be optional in the email address username, and if the first number in the subject/body matches your PIN, then it's removed and the post is allowed. People could choose their level of security vs. convenience. (not that access to the PIN buys an attacker much.... temporary spam in a person's journal, until the owner changes their PIN)