Sometimes, I sneak into an empty conference room when I've to really focus during the day.
If you are anything like me, the root cause may not be the noise and movement. It may instead be the anticipation that the noise and movement is someone who is about to pull you out of your flow. Even if it doesn't happen that often, the psychological jolt that occurs every time it does is enough to make my mind resist concentrating when potential distractions are around. Noise-canceling headphones do nothing against a tap on the shoulder.
From, you guessed it, Paul Graham... (Specifically, footnote #1 of chapter 6 of Hackers and Painters, aka "How to Make Wealth" http://www.paulgraham.com/wealth.html#f1n)
Aggh, I'm getting paranoid just thinking about someone walking in behind me now. Working when nobody else is around and no phones can ring is the best kind of work.