Here is a list of Apple iOS firmware (IPSW) official direct download links . We are actively updating the links . Links for iOS 4.3.1 , 4.2 and lower is coming soon .

How to restore / update firmware of your “iDevice”?
After downloading the desired firmware below, you can shift-click (PC) or option-click (Mac) the Restore or Update buttons in iTunes. A file dialog will open and let you choose the downloaded ipsw-file. (Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.)