Confuse the Word spell checker!
JavaScript, 71 characters

function f(s){return "\u202E"+s.split("").reverse().join("")+"\u202C";}


Try it out on JSFiddle.

By the way, this would be a good bit shorter in CoffeeScript (53 characters)

f=(s)-&gt;"\u202E#{s.split('').reverse().join ''}\u202C"


EDIT: I just noticed that UTF-8 requirement. That's going to be tough in JavaScript. :-/