On UBB I had a nice option to allow subscriptions to a board and give them the option to select either instant notifications or Daily Digest. We ran a nightly cron job that just cranked through the list of posts since the last invocation.
Incidentally this could also be used on an hourly basis to send everyone a digest of posts in the last hour.
I do have an interest in this - but I don't see how it works with very busy boards? Does it just send very brief info or does it send the post contents too? i.e. it could just say:
Lainaa
New Topics:
-------
Topic 1
Topic 2
New Replies to Topics You Watch:
3 replies in <topic name>
Or could include the post which could get quite long?
The one I've been using for the past few years would post for each board a full list of each post with a link back to the thread. That was the request of the members. They wanted a way to review everything offline dailiy and decide what to look at. Sounds like RSS to me :)
But I think if it had a list of new topic titles with a link to the post that would probably be good.