Two seconds after posting this, I renamed the Created field to "Created1" and copied the old Items tables field definition into the new table.
Then I went into datasheet view, and found both fields to be working.
Since I don't see how my actions could possibly have fixed this, I'm going to assume that one of you waved a magic wand. <grin>
Kidding aside, if you have any insights, I'd love to figure out what really happened.
Thanks.