I didn't see anyone posting anything very in depth on this, besides "farm low level herbs", so here's some help on exactly *which* herbs you should get for the 3.0.2 WotLK pre-patch, which will allow people to pick up the new profession Inscription.
The way Inscription uses herbs is by "milling" them up into a pigment, which is then used to make inks, which in turn used to make scrolls/glyphs. Herbs of a certain level will grind up into the same pigment, as follows:
Alabaster Pigment: Bloodthistle / Peacebloom / Silverleaf / Earthroot / Mageroyal
Dusky Pigment: Briarthorn / Swiftthistle / Bruiseweed / Stranglekelp
Golden Pigment: Wild Steelbloom / Grave Moss / Kingsblood / Liferoot
Emerald Pigment: Fadeleaf / Goldthorn / Khadgar's Whiskers / Wintersbite
Violet Pigment: Firebloom / Purple Lotus / Arthas' Tears / Sungrass / Blindweed / Ghost Mushroom / Gromsblood
Silvery Pigment: Golden Sansam / Dreamfoil / Mountain Silversage / Plaguebloom / Icecap
Nether Pigment: All Outland herbs
Azure Pigment: All Northrend herbs
This means that come patch time, all herbs in a given group should go up by about the same amount. The price should also be affected by how many of a given pigment is needed to go through a skill level range, in other words, by how much demand there will be for the herbs of that group.
While there is still a ton of changes being made to Inscription on the PTR, it seems that in general, you need more of the higher levels herbs that the lower level. This means that the biggest rise in price should be the high-level old-world herbs (they're harder to farm without having an epic flying mount), with not a very big rise in the very low level herbs like peacebloom (except in the first couple of days when people first pick up Inscription).
So, how can you turn this into profit? Do a little research at your auction house, and find what the cheapest herb in each group is. Then, pick a price, based on how much you're willing to invest before the patch, that'll you'll buy each herb at. Write this price down, or if you use auctioneer enter it as a bottomscanner snatch price. At least once a day, check the auction house, and buy out all the herbs that are in a given pigment group that are below that price. You should be getting a few stacks for each group every day, but try to get more herbs that turn into Violet and Silvery pigments, as those are the most needed, and should give you the biggest return.
PATCH DAY
If at all possible, log in as soon after patch as you can. If they announce what day the patch is coming out, you can do some of this too.
Immediately after the patch, prices are going to skyrocket. This is going to be the ideal time to unload much of your stock, especially all the low level stuff. There's going to be a ton of price variance, as someone puts up a ton of stock, then it all gets bought up by the people powerleveling, so you'll want to put your stuff up for a range of prices. For example, while Peacebloom might go for 20s a stack right now, right after patch, 20g might not be totally unreasonable. So, depending on much stuff you have stockpiled, put up a little bit for a huge price, a good bit for increments under that, and then much of it for just a bit over what is already available.