It was one of the "Save The Cute Puppies And Cuddly, Fuzzy Kittens Acts" of 2008. They barred the exits on June 17, 2008 before they lowered the boom on us after they got back from vacation:
The Heroes Earnings Assistance and Relief Tax Act of 2008 (Who could be against it? It's for the troops!)
New "Heroes" Law May Be Villain for Expatriating U.S. Citizens and LPRs Who Surrender Green Cards (02-September-2008)
Oh, that’s if you renounce citizenship. I’d never do that. I knew about that one.