One thing I’ve noticed in the new Cataclysm (that is, patch 4.0.1) is that you now gain experience from gathering professions.
I was only first really aware of it when I dinged level 32 after picking up some stranglekelp. I didn’t quite know what to make of it, I mean, Questing and killing were the staples of WoW and really the only way to gain XP was through that. Now it opens a whole new world of levelling possibilities.
With the opening of the cataclysm content I picked up Archaeology, the new Secondary gathering profession, and went off to gather some stuff from various dig-sites. I got to skill level 66 within about an hour or so of flying around, seeing the re-modelled world from the air as a bonus (it’s not that impressive to be honest, Trees still look like green Toblerone from above). I also found out that I still got full XP from digging up items, even at skill level 1. I went up half a level through 69 doing that, at 3000XP per node.
I haven’t tried it, but if you get full XP gathering items at lower levels (ie, they’re grey to your skill level) then it could be possible to level from 0-85 gathering nothing but peacebloom. If you don’t get XP from grey skill items, but still get XP when you hit the skill caps (ie, level 75 skill, 150 skill, 300 etc) then it might be possible to buy a skill, like herbalism, and when you hit 75 just continue to gather the low-level herbs that are still, technically, of worth to you, gaining XP all the way.
I can’t work out just how much you’d need to farm for it, but I’m guessing it’ll be slightly longer than killing boars.