Re: House: Series Wind Down Thread (Series Discussion, Spoilers!)
^^^ It was the approval from ICE for House's "wife" to become a citizen. This means she will be able to leave him, which was initially the plan that both she and House tacitly agreed to. The relationship seems to have taken on an odd dynamic and House now doesn't want her to leave, hence his throwing the letter in the trash, and Wilson's statement of him keeping her a prisoner in the preview scenes for next week.
I may appear unoccupied to you, but at the molecular level, I'm really quite busy.