Reading over Chief Justice Roberts' Medellin opinion, it reminds me a lot of Justice Stevens' majority opinion in Hamdan v. Rumsfeld in 2006. Recall that in Hamdan , the court blocked the Bush administration's effort to create military commissions unilaterally by reading the UCMJ as requiring Congressional approval for those commissions. The basic idea: If you guys in the executive branch wanna do military commissions, you gotta get Congress clearly and unambiguously on board first.
Medellin v. Texas strikes me as similar. The majority reads the court's precedents on treaties as effectively requiring a clear statement that the treaty is self-executing before it will be construed to be so. And it also holds that the executive can't act on its own and make the Avena judgment binding. The basic idea: If you guys in the executive branch wanna make these foreign judgments binding law, you gotta get Congress clearly and unambiguously on board first.
Of course, the people who like both Hamdan and Medellin could probably meet in a phone booth. (And Justice Kennedy is already in the phone booth.) But the two decisions seem pretty similar to me.