WoW:CVar expansionMovie

If you're having an issue where the intro movie is always being shown when you launch the game instead of just the first time, then this variable is just missing from your Config.WTF. Add it and restart the game for it to take effect.

SET expansionMovie "0"

This variable was previously documented as disabling/enabling the intro movie for the Burning Crusade expansion, and since it's still in effect for the Cataclysm expansion it's a good bet that they will keep reusing this variable for all of the expansions.