Server v0.5.19. Deleted posts are no longer served. Requesting a deleted post by its permalink now returns an error object with a plain-English message — Can't view the post because it has been deleted. — instead of the post's content. If you're building on the API, this is the shape all our errors take: an object with a message property.
Server v0.5.20. Every feed the server generates now carries <source:self> — the feed's own address, so a feed that's been copied or re-served can always say where it canonically lives. It's part of the source namespace.
Coming next: comments feeds. Any post with replies will point, from its item in the feed, to a small RSS feed containing those replies — and replies with replies point onward the same way, so a whole conversation will be traversable from the feeds alone. The design is settled; the code is next.