Having this same problem with ALL of my 4 WordPress sites - I try to run the "Automatically Upgrade" for 2.8.4 or for any of my Plugins, and it takes maybe 3-5 minutes for the system to give me an error message like this:
Downloading update from http://downloads.wordpress.org/plugin/google-sitemap-generator.3.1.4.zip.
Unpacking the update.
Could not copy file /public_html/wp-content/upgrade/google-sitemap-generator.3.1.4/google-sitemap-generator/sitemap-core.php
----I never had problems before WP 2.8, I see all people have the same problem, don't see a good resolution. I tried deactivating plugins one by one, was able to try the WP 2.8.4 update and actually got it to work!
But, alas, tried the above simple plugin update afterwards, got the same silly message! .....any help is much appreciated!
(I've tried CHMOD the file permissions to 777 - doesn't seem to help).