Looking deep into my crystal ball, I see... Wait, this is a magic 8 ball. Well, that sucks. I will have to use logic and reason instead. Yikes.
Well, here goes.
Frostweave. Bags and cloth. That's today's prediction.
When cataclysm launches, and for the next month or more afterward, frostweave bags will be pretty profitable again. First, people will want bags for their new goblins and such. Most people will remember their heirlooms prelaunch, but will forget to stock up on bags. Demand++
Second, almost the entire population of currently-80 toons will be headed to the new zones, where no frostweave drops. The new crop of alts will take a while to get to northrend, and once there won't stay for long. Supply--
But wait, you ask, what about the new tailoring bags?
I'm glad you asked. Simply put, the scarcity of new cloth, low number of powerleveling tailors, and demand for the new bag on 85 mains will price that bag well out of the range of alt supplies for a while.
But wait, you ask, what about netherweave bags?
Good point. My understanding of consumer psychology tells me most people will go for frostweave, but it wouldn't hurt to take a look at netherweave too. Watch both markets.
Right now, on my server, frostweave bags sell for about 45g. After cataclysm launches, I'd expect to see prices go up to 70-80 gold apiece. Much more than that and people will stop buying them in favor of cheaper smaller bags, but that's still enough room to play with the market and turn a tidy little profit.