This may be hard to believe, but I've actually leveled Herbalism three times in the Beta. The first time was on a pre-made character which I leveled from the ground up. Unfortunately she was already level 82 by the time Illustrious Herbalism was implemented, which led to the second leveling test. The second round was taking a level 80 through the same 450-525 range, which I was able to do successfully. I've also just completed a third test which I'll discuss at the end of this post.
When Cataclysm launches and you're ready to tackle Herbalism, you should probably decide where you're going first. The new zones are on two different continents, so switching between is a bit of a time sink. The nearest trainers for Mount Hyjal farming are probably Darnassus/Orgrimmar. If you want to go to Vashj'ir you should train in Ironforge/Undercity. I believe Mount Hyjal is preferable to Vashj'ir for a couple of reasons. First, you don't get a seafaring mount until you do the initial quest line in Vashj'ir. This takes about a dozen quests to unlock, meanwhile you're swimming about like a noob. This is fine for questing, but annoying for farming. Second, Vashj'ir is a three dimensional zone. You have mobs above and below you, in addition to the normal aggro radius. Some folks have trouble adjusting to this concept. Even if you don't think that will be you, sometimes mobs have the nerve to lurk behind huge blobs of seaweed where you can't even see them. Eventually we'll all adjust, but you may not want this much novelty on your plate right off the bat.
The herbs that spawn in the two entry level zones require a skill level of 425 and turn green at 475. The logical progression after that would be to travel to Deepholm and collect Heartblossom, the herb available at 475 skill. Unfortunately Deepholm is inaccessible to level 80/81 players, so our progression needs to provide an option that skips Deepholm. Luckily it's possible to farm Cinderbloom & Friends all the way to Whiptail, which requires 500 skill and spawns in Uldum. Whiptail can only be found near the river banks, so be aware that wandering the desert is an exercise in futility. You'll have no problem getting to 525 with Whiptail, it actually stays orange all the way up. Once you cap Herbalism you can also collect Twilight Jasmine in the Twilight Highlands, the final Cataclysm herb.
With my level 80 test I went to Mount Hyjal for the early portion of the leveling process. This is a workable plan if you haven't quested through Hyjal yet. As I discovered earlier today, it can be frustrating if you've completed most/all of Hyjal before you get around to working on gathering skills.
Phasing is not your friend as a gatherer. At this point I've doing a good amount of mining and herb farming in Mount Hyjal. In general it's much easier to go about your gathering before quest areas are phase-shifted. In one particularly nasty twist, a significant section of Hyjal becomes populated with aggressive elite dragons who will continue zapping you for 2-3k from half a zone away. After attracting the attention of one... then a second... of these babies I booked it to Winterspring. They continued to blast at me several more times before they realized I had actually gone to a different zone. Perhaps these little snags will be corrected before launch. Meanwhile I know where I won't be leveling my gatherers.
The only portion of Hyjal that was changed for the better was "The Regrowth". If you complete a set of quests that transforms this fire-blackened area into a budding forest, you should get some decent herb spawns in that section. I would absolutely not quest through "The Scorched Plain" or any other area south or east of that (Gates of Sothann, Seat of the Chosen, Forge of Supplication, Doom's Vigil). Well, assuming you want to preserve a part of Hyjal for gathering duties.
Which brings me to my latest leveling attempt - a dry run of the "power level" process to see what's involved in making that work. I know some of you are interested in going for Realm First, so I thought I'd try that out. I started in Hyjal, but I'm not sure that's the smartest way to go. Theoretically you could also go straight to Uldum and farm Cinderbloom until you hit 495, then start picking Whiptail. Uldum has both herbs, Cinderbloom in the sandy desert spots and Whiptail in the river bank areas.
But all of the new herbs turn green at 475 which means you'll have to get 20 additional skill points on them before you can switch to Whiptail. Leveling 20 points on green herbs seemed painfully slow to me. So I checked in with Wowhead and realized that Icethorn stays yellow longer than Cinderbloom/Azshara's Veil/Stormvine. Stupid as it sounds, it actually worked better to level part of Herbalism in Northrend.
I spent 22 minutes in Mount Hyjal getting my Herbalism to 472. Then I hearthed back to Dalaran and flew to K2, where it took me only 14 minutes to hit 495. What it boils down to is that you'll get more reliable skill-ups from Icethorn than the new herbs. The spawn density is also better in Storm Peaks and there's no competition from other players. So if speed is important to you, I have to recommend Northrend as the better leveling location. At 495 (with your +gathering gloves) you can portal from Dalaran to the Caverns of Time and be just one zone away from Uldum.
The ideal scenario for a Realm First attempt would be to have a Dalaran ring and your Hearthstone set to the classic city of your choice (to avoid being trapped in Dalaran). If you don't have a Dalaran ring then just leave your hearth set in Dalaran. You can learn the Illustrious rank of Herbalism in Dalaran and then fly to your preferred Lichbloom/Icethorn farming spot. Once you hit 495 skill (you have those +gathering gloves right?) you ring-port back to Dalaran and use the Caverns of Time portal.
Next, fly directly to the center of Uldum but don't use a flight path. You won't have the right ones anyway. Once you see the river, start collecting Whiptail. It took me about 12 minutes to hit 525, but I started at 500 skill instead of 495. You can probably assume 15 minutes if you don't have other competition in the area. There's a good amount of Whiptail along the river, but watch out for crocolisks and the occasional cat-dude.
All told it should take less than an hour to hit the Herbalism cap. If my numbers are reasonably accurate you can do it in under 45 minutes. If for some reason you don't manage to get Realm First, the consolation prize will be making mad money on all the Whiptail you collected. You can also head over to Twilight Highlands and score some of that Twilight Jasmine.
Bear in mind that this information could change between now and launch. To be honest, I would not be surprised if they tweaked the Cinderbloom/Azshara/Stormvine triumvirate. I find it strange that they can all be harvested at 425 with the other three herbs spread across 475/500/525. They could also shift Lichbloom to be green at 450. Plus, the CoT portal might be removed from Dalaran. I'll try to do another test for this if I get a chance towards the end of Beta, but no promises on a fourth go ;)