The iPhone browser is awesome and to make surfing really quick the keyboard (when typing in the mobile safari address bar) shows a .com shortcut button. But the www is not all about .com. There are other extensions that you would use regularly.
So here is the trick. Just hold down the .com button a little longer and you will see .net, .edu and .org buttons. Slide your finger from them and then release your finger to choose. Easy!