If you want to make some quick gold on launch day, keep a mage in Dalaran.
I expect the portals to vanish in either 4.0.1 or 4.0.3, so it will be old news by the time the level cap gets bumped.
On that note, remember that Cataclysm is deployed in at least 3 stages with money to be made in each. In 4.0.1 we get new talents and mechanics. This is when everyone will need new glyphs, and find out that all glyphs will require 2-3 times as much ink. Vanishing ink will be consumed by the barrel. Arp and hit gems change color. Enchants will change in either effect or value. This patch will get a lot of people playing their old toons. If you plan to make money off of guild achievements, this is presumably the time for that too.
Blizzard promises 4.0.3 to be the event called the Cataclysm, when Azeroth changes. Even without goblins or worgen, all the lowbie content has been redone so people will level alts. This is when the price for lowbie mats will crash, but the market for twink supplies will be lucrative.
Cataclysm proper is announced for December 7th, which is when we return to our mains and the 81+ markets open up.