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stackoverflow.com - Using an ORDER BY in a subquery is not the best solution to this problem.
The best solution to get the max(post_date) by author is to use a subquery to return the max date and then join that to your table on both the post_author and the max date.
The solution should be:
SELECT p1.* 
FROM wp_posts p1
INNER JOIN
(
    SELECT max(post_date) MaxPostDate, post_author
    FROM wp_posts
    WHERE post_status='publish'
       AND post_type='post'
    GROUP BY post_author
) p2
  ON p1.post_author = p2.post_author
  AND p1.post_date = p2.MaxPostDate
WHERE p1.post_status='publish'
  AND p1.post_type='post'
order by p1.post_date desc
If you have the following sample data:
CREATE TABLE wp_posts
    (`id` int, `title` varchar(6), `post_date` datetime, `post_author` varchar(3))
;

INSERT INTO wp_posts
    (`id`, `title`, `post_date`, `post_author`)
VALUES
    (1, 'Title1', '2013-01-01 00:00:00', 'Jim'),
    (2, 'Title2', '2013-02-01 00:00:00', 'Jim')
;
The subquery is going to return the max date and author of:
MaxDate  | Author
2/1/2013 | Jim
Then since you are joining that back to the table, on both values you will return the full details of that post.
See SQL Fiddle with Demo.
To expand on my comments about using a subquery to accurate return this data.
MySQL does not force you to GROUP BY every column that you include in the SELECT list. As a result, if you only GROUP BY one column but return 10 columns in total, there is no guarantee that the other column values which belong to the post_author that is returned. If the column is not in a GROUP BY MySQL chooses what value should be returned.
Using the subquery with the aggregate function will guarantee that the correct author and post is returned every time.
As a side note, while MySQL allows you to use an ORDER BY in a subquery and allows you to apply a GROUP BY to not every column in the SELECT list this behavior is not allowed in other databases including SQL Server.

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