Cataclysm launch is the best time to farm

The people making real money will be the ones out there happily gathering the new ores and herbs so that the poor crafter saps like me can pay a huge premium for a very small supply.

Economy of scale?

Yes, economy of scale. A complex operation with 10 steps will take less time per unit if you do each step 100 times in a row, especially if all you have to do is fill your bags from your mail and click "smelt all" or some such. When you farm, you have to perform a highly specialized task that demands your almost complete attention. I can't farm effectively while I do anything else; whenever I stop staring at my minimap to reply in guild chat, I miss a node. I certainly can't watch a movie like I do when I'm
milling. Still, if there's enough money in farming, I'll farm.

Cataclysm changes things

In addition to the fact that gathering ore and herbs now
gives you experience, there will be a huge group of auctioneers racing to the new professions skill cap. These are people who believe that being among the first to get there means that they won't have to deal with competition and that they will be able to make insane margins on their goods. They're partly right, but if they were doing their math correctly, they'd realize that these margins had better be pretty stupendous if they're to offset the ridiculous prices on farmed goods right after an expansion launches.

I remember seeing
Cobalt Ore for several hundred gold per stack when I started mining in Wrath of the Lich King. And this was before the massive increase to the number of people who played the auction house with a crafting trade skill. I predict we'll see an initial price for goods of at least five times what they end up selling for in a few months, simply because that's what I saw in the last expansion. I suspect it may be even higher, now that there are so many more auctioneers who will be competing for farmed goods and trying to buy stock for resale.

Good times to be a farmer

Assuming Blizzard keeps the glider bots away, the launch of Cataclysm will be an amazing time to be a farmer -- until the initial gold rush dies off, at least. The amount of time this lasts will depend on how much farming is being done, as well as how many people are trying to level crafting skills. I suppose that it's actually more accurately stated that it'll be a bad time to be a crafter. Basically, crafters will be caught in a situation in which the mats for their goods will be expensive but gradually decreasing in price. This means that everyone who waits a little longer than they do will have, on average, a price advantage. Also, buyers know this and often end up waiting on purchases of finished goods because they know it's just a matter of time until the prices go down, especially since very few people will be forced into buying endgame goods until they're at level 85 and raiding (or whatever).

So during this gold rush when everyone is buying farmed goods for a premium to get a leg up on their competition, I'd recommend farming to anyone. I'm planning on doing it, and so are some of the people I compare notes with about this stuff. I'm going to go ahead and caution people on one thing, though: If you decide to farm during the launch, you would be well served not to use your own mats to level your professions. If you've made the decision to wait until materials prices lower a bit, you're losing the current inflated value of your goods if you don't sell them.

Again with the opportunity cost? Really?

Yes, really. If you farm a stack of Cataclysm ore that's worth, say, 500g that day and you use it to raise your professions by 10 points, those 10 points cost you 500g. If you wait a week and a half and the prices (while still spiking up to 500g) bottom out some days at 300g, you just "made" 200g by waiting a week. If those 10 points opened up a recipe that would be worth 200g (in the week, not overall), then they're worth it. Otherwise, just sell your stack and use the money to buy one and a half stacks in a week.