Latest US tax treaty update is available here, with a new section covering the developments on FATCA IGAs. I'm still not convinced they should be included in such a list, since from the US perspective these appear to be nothing more than competent authority agreements, presumably entered into under the CA's authority "to clarify or interpret" existing treaty & TIEA provisions. Certainly, it seems that no internal legislative procedure will be undertaken to get these in force in the US--they await only the internal ratification by FATCA partners.
I have seen absolutely nothing offered by the IRS or Treasury that explains the character of these agreements, so I am deducing that they are being considered CA agreements from what I've seen so far. But I'll admit the evidence is conflicting and confusing: for example, the IGA with the UK was apparently signed by a non-Treasury embassy official, and not the CA, so how could it be a CA agreement?
Fortunately, at some point the mystery will be solvable: I'll be able to confirm the legal status of the IGAs as soon as the first one enters into force, by checking to see if it lands in the pages of the US treaties and international agreements series. If it does not (as I suspect), then the IGAs really belong only in a list of competent authority agreements, where they will be treated not as treaties or international agreements per se but merely as interpretations of existing treaties. That will be interesting because it really stretches the boundaries of how I think we've understood up until now the competent authority's ability to stray beyond the text of the treaty. Of course, even if they do not end up in the TIAS I can certainly understand why Connery et al included them in their regular tax treaty update, because they sure look like treaties, don't they.
If I am wrong, though, and the IGAs do land in the treaties and international agreements series, then things get even more interesting. We will be witnessing an unprecedented first in the history of US treaty making: the introduction of sole executive agreements on tax, not pre-authorized by congress, not expressly authorized by any existing treaty, and serving to override existing statutory tax law without any congressional oversight at all. Intriguing to say the least.
Of course, none of this has any bearing on the legal force of the IGAs from an international perspective. In the eyes of the world, these are just like any other international agreement. But in terms of those who think about the treaty making power in the US, I would think this would be a very interesting and controversial development.