Recently, Io9 ran a story about the possible last words of David Tennant's Tenth Doctor.
By taking clues from a page of the 2010 Doctor Who Storybook (see it here), they are speculating (as do some fans) that his last words could be ''Don't forget me.''
If you read the lines from bottom to top, you can see references to his regeneration from 9th Doctor to the 10th with ''Hello, ooh, new Teeth.'' Then, amongst others, he speaks of Rose Tyler, the Agatha Christie adventure in season 4 The Unicorn and the Wasp, Silence in the Library, Planet of the Dead, his upcoming appearance in the Sarah Jane Adventures and Waters of Mars and so on. It is also believed that he makes a reference to the Time Lords with ''Can't be alive! No! The gate!''
So if David Tennant's last line as the Doctor is ''Don't forget me'' as it appears at the top section of the page, you can believe me, many will shed tears. And no, no one will ever forget you David!