It turns out the Virginia's retrocession of their original land grant to the District (now Alexandria and Arlington) may not have been constitutional. Via the DCist:

As part of research Schiller has been doing of the historic archives of local newspapers, he found one article from 1910 laying out the case against the constitutionality of returning then Alexandria County (which encompassed today's Arlington County and the City of Alexandria) to Virginia and another article from 1890 detailing a movement of Alexandria residents (90 percent of them, it claims) demanding that their land be returned to the District. "The reasons for going back to the District are practical ones and appeal to common sense and business interest," said a petition on the matter. "If a vote was taken on the subject, nine-tenths of the people would vote to go back."

So we could have our perfect 100-mile-squared diamond back, but it wouldn't make us happier.