No. Posterous is really cool and I think it will be a big hit, but sending an email is not simpler than using the Tumblr bookmarklet, which makes blogging practically unconscious.
I hope that the posterous bookmarklet is good at keeping the layout of the things I select.
if someone is not already logged in their email, they have to type an address and then login. Is that easier from having a simple username/password signup page?
Also: if you can use email, you can use posterous. This opens it up to a lot more people who are afraid of "setting up" and remembering another password and confusing web interface.
So, those people who are not technical users, most probably use webmail. Even, if they do have another address (from their university) they would still probably login from the web or prefer not to use that address but that of their IM.
just my thoughts..
I'm guessing your audience will be tech savvy users who understand why publishing some conversations online using a tool you're already using anyway (email) is a good idea.
Not that there's anything wrong with that, of course.