It's expected that Cataclysm will bring about a surge of activity as people check out all the new content immediately after launch. Having played World of Warcraft immediately after The Burning Crusade came out, I know during the expansion surge, it's very hard to get anything done as you're competing with everyone to do the same starting content.
How long does this initial surge last, and when does the game back to normal levels (in the sense that you can complete quests and access NPCs without much competition)? A week? A month? A few months?