The iPad already has the spell check feature and now one of the features I have aways wanted for the iPhone and iPod Touch has made it. When you type something wrong you usually get it fixed with the built in correction software but sometime you may spell something wrong and it does not pick it up. Now in iPhone OS 4.0 your misspelled word will be underlined in red. When you double tap on the underlined word a menu will popup that gives you the options to cut, copy, paste, and replace. If you press replace it will give you the correct spelling of the word or alternative options of what you were trying to spell.
Just sharing a cool new feature I noticed in the beta. Also another cool feature is that you can sync notes with individual mail accounts!