With the upcoming 6.0.2 patch, unused character names will be released for use by other players. This applies to any characters that have not logged in to World of Warcraft since November 13, 2008. If you have not logged in to the game since then, your character names will become available for other players to use. You won't lose anything, but you will be required to rename your characters after the reclamation is complete.
If you want to make sure you keep your character names, log in to World of Warcraft and enter the game on each of your characters before the 6.0.2 patch is released. Logging in on a character will flag that character to retain its name.