Background of this problem
It happened when I created a post with this very blog.
I scheduled a post and then clicked the preview URL. The page was a 404. Every other post worked fine until now.
My configuration for Permalinks was a custom structure.
I set the Permalinks back to the default structure (http://www.gregpike.net/?p=123). Saved changes.
Then set it back to my custom structre.
My guess is the .htaccess file needed some massaging, though I didn’t touch it directly.
Edit: Someone with similar experience.