Thanks to the very helpful geekgrrl on the WordPress.org support forums, clock is ticking once more, after being broken for a bit. All posts have their content once more.
I noted the problem this morning with this note “Post content and comments not rendering (WP 1.5.2)” and a similar post “All copy in posts has disappeared had the solution.
Deactivating the Markdown plugin, which is one of the default set in WordPress, fixed the problem immediately: all content reappeared. I’m still running v1.5.2 of WordPress. I have no idea if this affects other versions and combinations.
I don’t believe I ever activated the plugin, though it’s possible it was on by default. I’m curious whether Simon Waldman‘s experience was triggered by # of posts or is all of this somehow strangely tied to the date?
I’m going to reach out to the developer of the plugin, Michel Fortin. No blame, but maybe he can take a look at his code if similar reports confirm the cause.