Downloading update from http://wordpress.org/wordpress-3.0.3.zip…
Download failed.: Could not create Temporary file.
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.