After some snooping around in Wrath of the Lich Kings beta files a button named "Paid Character Customization" was discovered.

WoW Production Director J. Allen Brack initially hesitated but then went on to explain that they haven’t quite figured out exactly what they would offer gamers yet.

In any case it looks likely that subscribers who are willing to part with a little more of their cash will get to customise their avatars. Perhaps they’ll let you mess around with colour palettes for armour and the like ...who knows.