Downloading update from http://wordpress.org/wordpress-3.0.3.zip…

Download failed.: Could not create Temporary file.

Installation Failed

Quick solution for that case (choose only one) :

1. Create upgrade directory under wp-content

Open ftp client and access wordpress directory

right click and choose “make directory”

enter the name  “upgrade” and click OK

chmod new folder “upgrade”  to 777.

2. Chmod wp-content directory

Chmod folder wp-content 777 in recursive and change back to 755 after upgrade process.