I finally had the chance to play with 0.2 a bit. First off, I dig that you can see your mail history. I didn't know that was a past option. I also like how there are different categories of email newsletters, although the term "category" threw me for a loop. I had to try it out to see what it did. Because I'd love to see Courier eventually as the tool for managing all of my subscriptions (including custom RSS, SMS alerts, etc.), I think "Edition" might be a better word?
I managed to figure out how categories work by creating my own, trying to send an email, and then seeing that it referenced a theme that didn't exist. For me, the ideal theme system would be server-side files for the styling and layout, and then I'd be able to choose the number of stories, their positioning, what image went in the top slot, etc. from the admin. I haven't looked at your code at all though so I don't know what sort of complexity this would add to the system.
Another idea for further down the road would be the ability to prepare an email and then schedule a delivery for the middle of the night (or any specified time). It would also be great to be able to pull in, say, the most recent/active forum posts or links saved with P2.
Looking good! You're making significant progress.