The free iOS4 update for iPhone 3GS, iPhone 3G and iPod Touch is out, you can upgrade to the latest version by using the iTunes upgrade option. If you do not know how to upgrade to iOS4, read our earlier article on How To Upgrade to iOS4 on iPhone 3GS, iPhone 3G and iPod Touch.
The new iOS4 update is a free update for the above devices, however, multi-tasking and wallpaper support will only be available for iPhone 3GS and compatible iPod Touch devices, before you upgrade, read about the battery usage with iOS4.
The iOS4 update is 366.4MB in size unlike earlier versions which were larger. iOS4 contains over 100 new features and fixes.
Also, though the iOS4 update is officially available right now, it would take some time to roll it out all across the world, so be patient and keep checking for updates and yes Apple servers will be slammed today, so expect for slower updates.