The political problem of a Tobin Tax is broader. As financial transactions become ever more sophisticated, there will be an increasing need for international cooperation. For instance, country A will need information on the holdings of its own citizens in country B's banks, requiring supranational overrides to national sovereignty-based bank secrecy laws. Cooperation will need to include sanctions, mandates, etc., merely for nations to be able to tax capital income. Thus I expect there will be more political pressure for arrangements that would also make a Tobin tax more feasible.
If places like the Caymans don't cooperate we can always remind them about Grenada.
MBS