Dave Winer asks about automatic unsubscribing from a feed, and it's an interesting question. If a feed is no longer updated, how should the publisher tell aggregators to unsubscribe from it? I think the simplest solution is for the server to return HTTP status code 410, as recommended here and here. An aggregator that receives a 410 for a specific feed should then automatically unsubscribe from it.
This will be supported in the next build of FeedDemon, and I believe it's already supported by several other aggregators. In FeedDemon's case, the user will be prompted to unsubscribe, since automatically unsubscribing would remove the feed from FeedDemon's cache - losing the user's history of that feed's postings. If the user chooses not to unsubscribe, FeedDemon will disable updating for that feed.