Of course, since there are fewer people playing while we wait for Cataclysm, this demand might be spread out a bit. Still, this will be the source of demand for glyphs going forward, and the big surge in demand will be when patch 4.0.1 launches. Also, since there are a fair few "completionists" who play this game, the demand won't only be for the "right" glyphs. In fact, for any class whose current best glyphs don't change much, most of the demand will be for the glyphs that have not seen much playtime until now.
Just be aware that if you're making glyphs (or buying them from undercutter-bots after planting some "seed" auctions for 3g each), you should try to avoid the ones that are being removed from the game. They'll be turning into Charred Glyphs, which should vendor for 50s. Also, remember that after the patch launches, glyphs should cost triple the mats they currently do. Stock glyphs, not ink. Lastly, be aware that many of the current glyph sellers are going to be dealing with a bunch of broken addons. Don't forget to pick up Zerotorescue's addon that will prevent QA3 from crashing into a steaming pile of LUA errors if you sell glyphs. Auctioneer will eventually get fixed, but QA3 looks like it's being left to stagnate by its writer, and without Zero's fix, many glyphs wouldn't be available. Not cheaply, at least. [Edit: apparently Zero is going to take over the development of QA3, which will have to be given a new name]
The next major change is that many classes will no longer want to gem for whatever they currently gem for, and all armor pen gems are being turned into the usually much less desirable crit rating. Luckily, the alchemy transmutes are not going to have a cooldown, so the price for epic gems shouldn't be much more than the equivalent rare gem. It will probably settle down to the price of the rare gem, plus the eternal needed for the transmute, plus whatever markup your jewelcrafters are putting on to cut gems currently. I suspect that the demand will probably not outstrip the supply, though, because every single auctioneer on every single realm seems to have been prospecting Saronite day and night since they removed the transmute cooldown on the PTR. I'm hedging my bets -- I'll only transmute and cut when I can make money, and worst-case scenario, the rare gems I have will be the best gems that you can put into pre-Cataclysm gear (as Cataclysm gems will have a gear level requirement).
If you plan on regemming your nice shiny level 80 epic leveling gear, take a look at epic gems if you have an extra 30 percent or so to spend on extra itemization that will get replaced by your first quest blue at level 81. If you sell these gems, don't despair if your stock goes unused immediately, as there will still be demand from leveling characters going forward.
Oh yeah, leveling characters ...
That's right, people leveling trade skills or alts are going to continue to be the best source of niche markets in the game. Patch 4.0.1 doesn't change much for them, except to remind them of how close we are to the expansion. One change that may affect the people gathering those old-world mats for them, though, is that gathering herbs and ore now gives experience. It's not a drastic game-changer, but if you like to gather on your alts while you level them so you don't have to buy ridiculously overpriced mats to level whatever profession you want for the level cap, you'll be getting a little experience.
Leveling characters are probably the only growth market while we wait for the expansion. BoA enchant scrolls, trade skill leveling kits, bags, raw old-world mats and finished goods like enchanting rods will continue to sell well.
If you're a long shot, risky-flip type, be aware that the Tome of Polymorph: Turtle may well be taken out of the game in Cataclysm. Or it may be put on a reputation vendor. Who knows? Regardless, Zul'Gurub is definitely being taken out of the game, and many people who want the soon to be unavailable achievements associated with it are buying the drops to get exalted with the Zandalar Tribe. Farming for them is actually quite profitable, and if a tome drops off the optional boss, even better.
Personally, I barely have time to keep up with my normal business, so forget about farming ZG. I have made a killing off tome speculators, though. I'm lucky enough to have someone on my realm willing to buy every one of these under 5,000g, so I've been buying anything under that to relist at 4,999g. Every single one has sold to the same guy.
Insider Trader takes you behind the scenes of the bustling subculture of professional craftsmen, examining the profitable, the tragically lacking and the methods behind the madness.