I was at the Apple Store a few days ago, and started watching the info video behind the Genius Bar. It showed that the words in a message that you are going to send could be italicized, underlined, or made bold by tapping the word twice and choosing the action that you want to use. I tried this on my iPhone 5 today, and it didn't work. It didn't give me the options that I mentioned, only cut, copy, paste, replace, and define. I am on iOS 8 beta 3, and I don't know if that matters or not.
Edit: Nevermind. I think that it only works on e-mail messages.