Incoming wall of text... first post, hope detailed guides are liked here. >_>
I can't disagree more with this suggestion. There are three major prof changes coming in Cata that make this a bad idea, with one caveat.
First, they are making prof leveling much quicker. Players leveling profs will get multiple prof skill ups per craft for higher value goods. Blue items will typically give 5 skill dings when initially learned. So profs will use, on the margin per ding, fewer mats. Now, this is the critical point, for leveling toons this will have a non-linear, highly disproportionate effect on demand!
There are two types of prof levelers: those who level profs as they level and try to have their profs keep up with their leveling, and max level toons who powerlevel profs from 1 to the cap in one go. The second type of prof leveler is totally irrelevant to initial demand, since people interested in their mains are going to be powerleveling to 85 and/or leveling their prof from 450 to 525. Nobody is going to bother dropping profs until at least they hit the cap since there's more interesting / better things to do.
This leaves just levelers. Now, most players who level profs on their toons as they level pick a gathering / crafting pair. Traditionally, this has meant that the crafting prof has fallen behind because of the much higher mat needs for level the crafting prof than the gathering prof. With multiple dings per craft this will no longer be the case! So, instead of everyone leveling in sync buying ~half the mats needed to level from the AH, they will be buying a smaller fraction from the AH. Thus, even if the number of people doing this doubles, there will likely still be lower demand in aggregate.
Second: Flying mount in Azeroth. Old World mats have held their value in TBC and WotLK because they are needed for leveling profs, but they are very inefficient to farm. Flying mounts in Azeroth will fix this. Say you're looking at Briarthorn at 80g/stack right now. If you're buying that expecting to flip it at a profit, lets say your target sell price is 160g/stack. How many people do you think are willing to pay that much for a stack of Briarthorn if they can just fly over Silverpine for 5 mins and gather that much?
In other words, supply of old world mats is going to shoot through the roof.
Third, data mining data has found a BoA tome that doubles the number of skill points you get for leveling profs! The speculation right now is that these tomes will be purchasable by anyone with a max level prof to make releveling the prof much easier for all future toons. I don't know about your server, but on my server a very large percent of herbs used are from people leveling extra alchemists for xmute alts. These players will now need half the herbs as previously.
Caveat: If you are planning on flipping your entire stockpile of old world goods in the first couple weeks of the expansion, then you may do okay, but even then it's not a sure thing imo. There may be some people who haven't used their DK slots yet that want to level a Worgen or Goblin DK, and if they're the type of player who likes to keep profs in sync with their level, they'll need to powerlevel from 1-275 when TBC recipes will start. This small set of players may lead to a temporary spike in mat prices.
So, in conclusion, demand will plummet (due to speeding prof leveling) and supply will skyrocket (due to flying mounts in Azeroth), so there is no possible sustainable gain to be had in stockpiling old world mats.
*whew*
So, what should you be stockpiling?
Answering this question depends on your investment goals. Are you looking to preserve capital against inflation or flip some goods for small profit from Cata changes?
Deciding which you want to do has wildly different investment strategies.
First, lets cover inflation, and inflation protection since it's simplest.
I'm just going to jump right in and assume you're on board with the idea of general gold inflation in Cata. Simply due to the power curve, Cata content needs to reward much more than WotLK content otherwise people would just farm the WotLK content with gear that trivializes it. I'm personally expecting the traditional 5× inflation over the first year of the expansion, with a minimum 100% inflation by the time most people start hitting 85.
For example, with my ~400k nest egg, that makes inflation kinda a big deal. If I just sat on that, I'd be looking at the equivalent in today's terms of 80k a year into Cata.
So, how do you combat inflation?
Vanity items. By far the best store of value in WoW are vanity items. They have low turnover, but that's okay, since if you're actually just concerned about protection against inflation then you're okay taking your time liquidating at the other end.
The best example of this is tracking White Kitten prices since Vanilla. It has had the exact same supply for the entire time. I remember when they sold for 1g in Vanilla. Now they fetch 100g easy. Expect them to sell for 300-500g in a year.
If you can get your hands on them, the Sealed Chest vanity items from the Shadowmourne completion rewards are killer for protecting against inflation. These are vanity items that, right now, are created incidentally from guilds pursuing top tier content. In Cata, these vanity items will only be pursued by (imo crazy) people looking to pursue vanity items! (SMourne itself being a vanity item). I'd suggest paying up to 100k for the mount, 40k for the tabard, and 20k for the other three.
Other good options are Tourney pets for 400g per or less, rare drop pets for 1.5k per or less, white kittens (yes, srsly) for 100g per or less, or hyacinth macaws for 10k or less. The only exception is the azure drake pet, since the mobs that drop it are being removed from the game, this pet is a total wild card.
OK that covers inflation, so what should you invest in if you're looking to actively profit from Cata changes? WotLK 350-425 mats are by far the best bet. Yes, even with the prof changes listed above.
The reason is two fold: first and foremost, margin of safety. Saronite Ore sells on most servers for about the vendor price (if smelted). JCs are mass churning amulets out of green gems to vendor. Bot farmers are farming the hell out of WotLK level herbs, driving prices into the ground.
Secondly, there is no way that anyone in Cata will be farming these in the volume that it happens today. In particular, it won't interest bot farmers since WotLK mats just won't have the volume.
So, they're currently trading for ~vendor costs, and we know that Cata ore is going to have higher vendor costs (either directly, or through tradeskills), and the supply is going to plummet. The only way prices can go is up.
Also, bags are a great choice. The OP was bang on there. Bags will always do well at the beginning of an expansion. I'm personally planning on putting about 20 frostweave bags up on the AH per day for the first week.
There are a couple more possibilities for Cata investment, but I've run out of time to go into them. I'll try to come back and post more later.