FTP upgrade not working

To make it easier for clients to upgrade WordPress on our server, I’d prefer to do it through WP’s own interface. IOW, they can click on an Upgrade Automatically button (to upgrade WP) or upgrade the plugins from the plugin section clicking on the links therein. I’ve been doing this manually, but with almost a dozen installations of WP under my belt, doing upgrades manually is not going to work anymore.

I first tried entering FTP information when prompted, but nothing happens…the browser sits there and does nothing. Next, I hard-coded the FTP information into wp-config.php, and again, nothing.

define('FTP_HOST', 'ftp://myhost.com');
define('FTP_USER', 'my_username');
define('FTP_PASS', 'my_password');

I then found a support ticket #10369, here:

http://core.trac.wordpress.org/ticket/10369

What you need to do is add the following lines to config.php in order to change some of the constants:

define('FTP_BASE', '/path/to/my_web_root/');
define('FTP_CONTENT_DIR', '/path/to/my_web_root/wordpress/wp-content/');
define('FTP_PLUGIN_DIR', '/path/to/my_web_root/wordpress/wp-content/plugins/');

This is needed when your FTP login does NOT put you in your web root, as mine does. At some hosts, you have separate home directories and web root directories. Defining these constants points FTP to the proper location of WordPress.

Or rather, it should. I now get a little bit further in the upgrade process: the new version of WP downloads, completely unpacks, but fails when the script starts copying the new files to my WP location:

Downloading update from http://wordpress.org/wordpress-2.8.6.zip.
Unpacking the update.
Could not copy files
Installation Failed

There must be something else I can define in wp-config.php to make this work. If I’m logged in via FTP under my own user ID and password, I have full access permission to the files, so there is no reason for this to timeout and fail. Looking in the …./wp-content/upgrade directory, I see the newest version uploaded and completely unpacked. So, something during the file copy process is balking. I peeked at /wp-admin/upgrade-core.php but did not immediately see anything I could define to correct this. One thought: is WP staying in FTP mode when it goes to copy the files?

Any ideas? I’m probably missing one step. I don’t want to change ownership or file permissions–this upgrade should be possible through FTP alone.