Fan site WarCry attended a Q&A session during last weekend's BlizzCon '08 and came back with this:
When asked to expand upon a button found by sifting through the Lich King beta's data files named "Paid Character Customization," [World of Warcraft production director J. Allen] Brack initially hesitated to give any answer at all. Several questions later, he went back to the matter, saying that he could, in fact, confirm that World of Warcraft would eventually have some form of paid character customization, though they themselves hadn't yet worked out any details.
Inhabitants of the World of Warcraft can rest assured that their new hairdos are likely free, as the release of the Wrath of the Lich King expansion looms near while the paid character customization is evidently still in the conception stages. However, the more costly makeovers could arrive in a future patch or the next expansion after Wrath of the Lich King.