A Treasury representative speaking at the ABA Money Laundering Enforcement seminar in October indicated more that 1,000 banks had registered and that they expected to publish the list after the first of the year.
As noted, the registration is voluntary. It must also be renewed annually. As the instances where sharing is allowed must involve terrorism or money laundering, it's difficult for me to see the value to banks who are more likely going to want to talk to another bank about kiting or some other type of multi-bank fraud.
If presented with a situation where they want to share on an appropriate fact situation, banks can register instantaneously via the FinCEN web site. In short, I do not see the incentive for advance registration and the acceptance of a responsiblity to renew annually. (We'll see if the examiners pressure institutions to register "just because they should.")
Personal opinion only: I think the most likely use of this system will actually be prompted by law enforcement requests. For example, if a bank files a SAR on money laundering which mentions another bank and Treasury is interested, they will "suggest" that the two banks register and share information in order to show law enforcement the big picture.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.