Though related, this is no longer true. You can now purchase a lvl 90 character boost. Some power levelers have hoped for a day when you could use real money to buy individual items in-game, some times called "micro-transactions." At a Game Developers Conference 2008, Rob Pardo gave a strong indication, Blizzard would not ever go this route with World of Warcraft.
90-91 i collected treasures that gave resources and then dumped them all in xp potions and began to clean up shadowmoon valley while also doing bonus objectives, after shadowmoon i went to gorgrond and did the same thing and then to talador for a bit before I was 96 and went to quest in arak until I was 98 when I picked up my artifact and went back for the last few levels. Overall 90-100 took like 4 hours.