Last time this was suggested, some arguments against it were made that made sense to me (though I can't remember what they were). However, it really bugs me, and I would love to have a way to keep the default (so it's applied for post-by-email and twitter posts) but be able to change it when posting using the form.

(Basically, I want the Twitter posts to go private so I don't annoy my friends, and the only way to do that is to make the default private, but I also want to be able to post something access-locked and have it trigger the notifications, which I can't currently do without changing my account settings every time.)