I just trained herbalism to 300 on my level 58 beta Death Knight. I figured I might as well post the results in case anyone finds it interesting.
I was equipped with the class 100% mount, but it didn't occur to me to spec Unholy for the 20% mount speed boost. For reference, I have leveled herbalism to 300 on the live servers three times (once when I initially leveled my character to 60, and then twice when I dropped it to disenchant old soulbound gear and then retrained it), and I also consulted leveling guides via the Wiki to sanity check my methods. (I do lean towards herbing in zones where I can pick everything, so that I don't get stuck with lots of spawns I can't loot, so I may have spent more time on green and yellow herbs than some guides recommend.) I would estimate that this took about 6 hours of played time; the actual time elapsed was significantly longer, but I did do some other errands to break the time up a bit.
Without further ado, the haul was as follows:
89 Silverleaf, 67 Peacebloom, 84 Earthroot, 29 Mageroyal, 27 Swiftthistle, 50 Briarthorn, 1 Stranglekelp, 60 Bruiseweed, 51 Wild Steelbloom, 6 Grave Moss, 55 Kingsblood, 26 Liferoot, 12 Fadeleaf, 41 Goldthorn, 28 Khadagar's Whisker, 59 Firebloom, 3 Purple Lotus, 3 Arthas Tear, 16 Sungrass, 37 Blindweed, 12 Gromsblood, 16 Golden Sansam, 15 Dreamfoil, 8 Mountain Silversage
Explored Elwynn, Redridge, Stonetalon, Arathi, the Searing Gorge, the Badlands, the Swamp o' Sorrows, the Blasted Lands, and the Burning Steppes. (Also would have fully explored EPL, but the map for DK's starts fully unveiled and I appear to be missing something.) Points for five rounds of training herbalism.
Number of times I had to listen to that infernal Deathcharger mountup noise: Let's just say that I'm really looking forward to a robochicken at 60.
Advice for any other DK's who want to try this feat: Wait for level 58 so you can get to Outland and access the TBC herb trainer. (Alternately, find a mage to give you a portal sooner, having your hearth in Shattrath is useful anyway.)