Leveling areas will be included in TBC.
    How Blizzard did this to separate the expansion packs from the main client “Classic” so you would level up your character in classic or you can bring a character over to TBC, in doing so will prevent that character to come back to Classic. However, the 1 – 60 guides from Classic should also work in TBC if that character is not yet 60, others in the forum had also said they have managed to use classic guides in TBC to level their low level character.
    I hope that makes sense and that answers your questions. :)
    @Dugi correct me if I am wrong.