Wordpress 2.8 got released two days ago, the fully-automated upgrade introduced in previous versions makes the upgrading task as simple as it can get, however, after upgrading to the latest version I started receiving following error when trying to update/install a plugin:
Downloading update from http://downloads.wordpress.org/plugin/pluginname.zip.
Unpacking the update.
Incompatible archive PCLZIP_ERR_BAD_FORMAT (-10) : Invalid archive structure
Further research revealed that this is one of the bug which got slipped in the latest 2.8 release, the simple fix is to download the fixed http.php file and install it over in wp-includes directory.