In Wrath of the Lich King, all spells are being reworked so that their cost, rather than being a static mana value, is a percentage of your base mana pool (this is largely to prevent downranking). For instance, in the current, live game, Levitate costs 100 mana. In the Wrath beta, Levitate costs 3% of base mana But what is base mana?
It's the amount of mana you have before talents, buffs, and stats (like intellect and +mana) are factored in, and before base intellect from your race is taken into account. In other words, it's the amount of mana you would have if you were naked, unbuffed, and un-talented, and had no Intellect. Thus, it is a static value for each character of a given class/level combination. Getting more Int from gear will not raise the cost of spells, because it does not raise your base mana pool (it raises total mana pool). [Thanks for the correction, Breck and Improbable.]
So to the person who wrote in asking "what's the point of getting more Int if it will just make spell costs go up," I hope that answers your question: more Int does not affect your base mana pool. Ultimately we should see little difference from this base-mana-pool spell cost change, aside from the death of downranking.