As you all know probly, the new profession, Inscription, will have the similar system as jewelcrafting. Prospecting an ore give you a gem, and using the new inscription ability u'll be able to get Ink out of 5x herbs.
So its as simple as that, farm all the herbs (even the low lvl ones, since peacebloom and silverleaf are going to be used in inscription recipes), and keep them. Once the WOTLK will release, put them on AH, the prices will be huge probly, as it was with gems / ores at TBC release. Also... prices of gem and enchanting mats will probly go up, since ppl will want to enchant / gem their new northrend gear.
Prices of the herbs already went up on my realm, peacebloom used to be sold for 30-50s a stack, now its like 1-1,5g, earthroot was around 1g, now its like 3-8g, depends on day.