Michael J. McIntyre (Wayne State) has posted United Nations Code of Conduct on Cooperation in Combating International Tax Evasion, Tax Justice Focus, Vol. 4, No. 1, p. 1 (April 2008), on SSRN.  Here is the abstract:

Stopping international tax evasion and abusive tax avoidance is an exceedingly difficult task unless countries act together in a cooperative manner. The League of Nations, which undertook in the 1920s to foster the goal of taxing income once and only once, recognized from the start that cooperative efforts were essential for success. A first step in getting international cooperation is establishing international norms of conduct that responsible governments are expected to observe in preventing tax evasion and abusive avoidance. The United Nation’s Committee of Experts on International Cooperation in Tax Matters, which is the spiritual successor of the Fiscal Committee of the League of Nations, has taken the first step towards establishing those norms by voting to approve in principle a code of conduct. This short piece makes suggestions for the content of a UN code of conduct.