This is "Meanwhile in the TARDIS 1", a minisode from series five set moments after the end of The Eleventh Hour. It was included as a special feature on the DVD release, but is also on Youtube:
Amy: Why is it a police box?
[Amy babbles a bit; I'll spare you]
Doctor: Well it's not really a police box. Which, by the way, is a special kind of telephone box that policemen used to use.
Doctor: It's camouflage. It's disguised as a police telephone box from 1963. Every time the TARDIS materializes in a new location, within the first nanosecond of landing, it analyzes its surroundings, calculates a twelve-dimensional data map of everything within a thousand mile radius, and determines which outer shell would blend in best with the environment. And then it disguises itself as a police telephone box from 1963.
Doctor Who Minisode "Meanwhile in the TARDIS 1"