Russell T. Davies originally wanted to have The Doctor and The Master swap bodies. However, Davies wasn't keen to spend time during David Tennant's final story with the actor playing anybody other than the Doctor, and was also mindful that he had used a similar notion in "New Earth".
This is the first time the villain is credited in the opening credits.
The Vinvocci were originally named Shanshay and Shanshay - the pronunciation of the two names being so subtly different that Wilf is unable to discern it (although the Doctor can).