One more thing I wanted to find out about. How do you handle email signatures? When someone comments on a task in BaseCamp (via email) the entire email is posted to the thread, including their signature and whatever little images they included (photograph, or Facebook/Twitter icons). Is there anything you can do about that? Gmail seems to be pretty good at detecting the boilerplate part of a message -- possibly only after having seen one message from the user. This would be an important thing to nail.
Edit: What if you want to lock a page down so that only you or a select few may email updates? I would probably want that for at least one of the pages. Now that I'm thinking about it, the lack of registration is possibly due to how hard it would be to authenticate a user via easily-spoofed email. You might be able to come up with a sort of "secret key" that you add anywhere in the email but I could see that not working out for your average user.