Some time later now, a few current details:
Since November 2009, one might get a topbar notification while editing:
Be advised: this post has been edited 1 time by other users while you were editing.
Nice! But indeed when two people save almost simultaneously/concurrently, then this warning is not shown (apparently it's updated every 30 or 45 seconds), and the last edit overwrites the previous one without any notification. (The revision history still shows it, of course.) This can even accidentally overwrite the "duplicate of" links.
The 5 minute grace period makes things even harder: when someone else starts editing within that grace period, and then the first author saves a change within the grace period but before the second editor saves, then no new revision is created. Next, when that other person saves, a new revision is created but the most recent changes are silently overwritten. (Like a post in which Martin changed "10.6" to "10.5.8" within the 5 minute grace period, which was undone when Mehper saved an edit that was started before Martin made that change, but was saved after that.)
(A "new revision" is actually split into title, body, and tags. So maybe the existing automatic merge of these components is the best we can expect? Even then: if someone changed just the title, then edits that someone else made to the body might not be accurate after all, so here a warning might be nice too. And I don't care if the last edit is blocked or can still be saved, but some warning might be great.)