Have you bothered to read the opinion yet? (If so, I'd guess that you are in a very small minority on this thread, but based on your comment, I'd guess 'no')
PDF here:
http://www.supremecourtus.gov/opinions/07pdf/06-984.pdf
"In sum, while the ICJ’s judgment in Avena creates an international law obligation on the part of the United States, it does not of its own force constitute binding federal law that pre-empts state restrictions on the filing of successive habeas petitions. As we noted in Sanchez- Llamas, a contrary conclusion would be extraordinary,given that basic rights guaranteed by our own Constitution do not have the effect of displacing state procedural rules. See 548 U. S., at 360. "
(slip opinion at 27)
This has been discussed and after relfection the SC set a precedent that may help Presidents avoid pitfalls in the future.
Presidents have the right to sign international treaties, they need to be ratified by Congress. But I think few would disagree that foreignors arrested in foreign lands should have the right to confer with their embassies.
What strikes me is how so many on these threads have turned this into a high five fest because they think they scored one against illegal aliens from Mexico. The question goes far beyond and is far more serious than getting a good slap in on a few illegals.