Since no one seems to be jumping on this, I'm going to have to hack it(in the truest sense) together. Here's the approach I've been thinking on....I'd appreciate any comments or suggestions.

1) Add a userfield to keep an array of the forums on a per user basis. This will keep a flag of some sort for each forum noting that the user wants a digest for that particular forum.

2) Modify forum bits to have a checkbox for each displayed forum that would allow user to request a digest for that forum. (I need some help here...what would be the cleanest way to do this? I would like it transparent to user...i.e. I'd rather not have to have them click some button to save the changes, it should be saved no matter what they do on the page).

3) A chron job will run daily to gather the text from new messages since the prior chron run...probably forum by forum for now....i.e. user would get one email per forum they have digest enabled for.

Step one, build the text for messages.
Step two, scan the user,userfied tables to see who gets this forum in digest mode and send email..pref. a large BCC for fewer emails going through SMTP

I'm still working through the best way to manage the forum flags in the forum display bits and the best way to represent the flag data in the db....I was thinking of a mask but don't want to complicate it if I don't have to.
Any thougts or other examples out there would be appreciated.

Anyone see probs with just doing a chronological forum display versus a forum/thread sort? If so, how would the later look in the email?