At this stage we only have 2400 people subscribed to the digest, so I am not sure it is worth doing segregation yet. I played with Feedburner email service. It has some limitations in layout, but it works. However, automated stuff isn't always the best thing to send to people.

I need to do more investigation into feeburner. It offers what seems to be the most popular rss->email service online and maybe we could automate the process of letting people sign up by email to our various feeds (i.e. if we can construct a link that automatically generates a Feedburner feed/email page we could just add that link to the category/authors header)

If anyone wants to research this please let me know what you find.