In the CouchChat example, if you delete the chat room document, everyone loses access to the channel that the chat message documents are in, and the chat will no longer show up in the UI because its document is gone. But the messages remain.
Currently, the client is not notified when it loses access to a channel, and it doesn't delete the documents. This is appropriate in some situations (my analogy is that "if your subscription to the New Yorker expires, they don't come to your house and take away your magazines"), but many developers have asked for the local docs to be deleted.
There's an issue in Sync Gateway tracking this (I'm offline right now so I can't look up a link) and I've done some implementation work, but it's actually a tough problem due to the way channels are implemented. We're hoping to add this in a future release.
—Jens