In holding that the reach of the Constitution is to be measured functionally, not formally, a majority in Boumediene resolves a question previously muddled by plurality opinions.
Whether U.S. agents must adhere to the U.S. Constitution when acting outside U.S. territory is a question various courts have answered in different ways. As I'd outlined here when Rasul was pending (Pages 295-99), a line of splintered decisions that I've called "maximalist" indicated that the Constitution always constrained agents abroad. A "minimalist" line indicated the opposite, and neither expressly overruled the other.
A close reader of Justice Anthony M. Kennedy's concurrence in one of the latter cases, United States v. Verdugo-Urquidez (1990), might have expected that when give the chance, he would reconcile the two lines with a midway approach; that is, by taking the lead of Justice John Marshall Harlan in Reid v. Covert (1956) (concurrence) and hold that whether the Constitution applied in a particular extraterritorial instance required careful reviews of all the circumstances.
And today, that is exactly what Kennedy did as he wrote for the court that "practical considerations" compelled extension of the constitutional privilege of habeas corpus to noncitizens detainees held at Guantanamo.