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?
Not everyone uses webmail. Also, it's a lot easier from a mobile device. Also, we hate using web forms because they're such a pain. I hate seeing a Browse button! Now I can drag and drop right into Thunderbird.
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.
I think you're being too optimistic about the number of non technical people that will use this service. For them to do so would require them to understand what blogging is, why they would want to keep a blog and how simply mailing your posts is an improvement over typing something into a web form. Posterous is a fantastic idea (IMO anyway) but I doubt this will happen anytime soon.
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.
it's a nice feature but I assume your audience is also mainstream users.
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.
