Apple released iOS for iPhone as well as iPod touch, this time only iPod touch 2G and 3G are eligible for iOS 4 update. Following are the offline download links for iPod touch 2G and iPod touch 3G. It can be upgraded to in iTunes by holding the alt/option key (for Mac users) or shift key (for Windows users) while pressing the "Check for Update" button in iTunes, and selecting the downloaded IPSW on your drive.
iPod touch 2G: http://appldnld.apple.com.edgesuite….3_Restore.ipsw
iPod touch 3G: http://appldnld.apple.com.edgesuite….3_Restore.ipsw