In the latest version of the WotLK beta drinking a potion now gives you a "potion sickness" debuff that prevents you from drinking any other potions until you rest out of combat for some seconds. Which means that even in very long combats you can only drink one single potion. That has zero effect on solo players, but casters in raids often drink several mana potions per fight.
I don't think this will hurt Alchemists very much. Yes, the demand for mana potions will probably go down. But what few people realize is that you don't actually make money with Alchemy (except for transmutes), the potions you make cost just minimally more than the herbs you need to make them. The real money is in Herbalism. And while Herbalism thus loses some demand for herbs for mana potions, they'll earn lots of new customers: Inscribers. As a bonus Inscribers are less picky than Alchemists, because they don't need one specific herb, but any of a similar level will do.
How raiders will survive without multiple potions is a different question, but they will adjust. Priests already use mana conservation and regeneration talents a lot, and get more of them in WotLK. Damage spellcasters will have to do the same. So don't be surprised if you see a Mage rolling need on some gear with a big spirit bonus. :)