This new feature was just rolled out. You can now tag your friend in your status by adding the @ in front of their name. As soon as you do that, you'll see a list of friends with similar names displayed and all you have to do is select your friend from the list.
After you select your friend it will add their name highlighed in blue as a hyperlink and the @ symbol dissapears.
You can also tag more than one person by just adding again the @ in front of the other name.