Spell check is a minor but very handy feature that comes with the iOS 4 update. Probably this is the very first time you know about this feature as it is a relatively minor feature compared with those imminent ones (such as multitasking).
The spell check is a system wide feature added in iOS 4 for both iPhone and iPod touch. In other words, you can use spell checking in most iPhone apps (such as Mail, Notes). Like Microsoft Word or Gmail, word that is considered as misspelled is underlined in red. You can tap on the word and iPhone will popup the suggested replacement. By tapping on the suggested word, the misspelled word will be replaced with the correct one.
You do not always need to tap on the misspelled word in order to popup the suggestions. Say, you have typed “Wating “. As the word is detected as misspelled, iPhone will automatically show the recommended replacement when you hit the backspace button.
The spell check also comes with a handy “Replace” function. Sometimes, the word is correctly spelt. However, you may want to have other choices of word. You can double-tap on any word and tap on “Replace…” in the popup. The Replace function will suggest other words for replacement.
The spell check feature is only available on iOS 4 or up. If you want to use the feature, go ahead to upgrade your iPhone OS.
Note: For iPhone 3G user, iOS 4 is not recommended. Some users have reported that they experienced a significant slowdown after upgrading to iOS 4. For details, you can check out this post.