Well, I'm not in beta, but I am keeping abrest of the general vibe from what people are saying about professions in the expansion, along with making some assumptions based on past experience.

Here are some of my assumptions, my rationale for each, and my market conclusions:

1) Around 30% of the player base will go crazy for the new profession inscription. - As happened with JC in TBC. However only about 15% of the player base actually pursued it long term.
2) Early indications are that inscription will require lots of herbs to level. - Check any of the major news sites and it's already pretty clear.
3) Some of the player base will plan ahead and collect enough materials to power level to 300+, the majority will not, and will hence either have to farm for themselves, or be subject to the whims of the market. - My strategy relies on the assumption that the majority will not plan ahead, so I will be able to price gouge to meet the rising demand as people rush to level inscription.
4) Ore and bars are used in 3 professions, all of which will continue to prove popular - Blacksmithing, Engineering and JC all look like they are they each are going to give some nice bonuses in Wrath, BS in particular is getting some nice new functionality around socketing weapons. I predict many plate wearers will reroll from previous professions to take up BS when they see new skills emerging.
5) Everyone and their brother will roll a DK, some will persevere and make them a main or a significant alt, others will give up. - There will be hundreds of DKs running around in the 1st few weeks of Wrath, DKs are plate wearers, many of these guys will go for the professions already listed above, raising furhter the demand for low level herbs and ores.

Be warned, however, the above are ASSUMPTIONS, the usual precautions as apply to any stock speculation in Real life apply here: "The value of your investment may go up as well as down." I guarantee I'll misjudge parts of the market and end up selling some stuff at a loss, but with a balanced portfolio of stuff to sell I hope to walk away with healthy returns. Also note that I've virtually stopped investing in stock now, I have 9 bank alts full of goods on 2 servers, prices are already beginning to change and are guaranteed to fluctuate wildly the closer we get to Wrath and the more people speculate on what will sell and what wont.

EDIT: Plus what silencer said... good advice and analysis too.