In the United States, with Circular 230, that went into effect June 20, 2005, any taxpayer who wishes to set up an aggressive tax scheme with the assistance of accountants, lawyers or any other professionals, must submit the opinion letter issued by that professional with a description of his aggressive tax scheme as well as a request for a tax rebate. This is necessary in order to obtain authorization for the scheme. If this is not done, or if the aggressive tax scheme is attacked, stiff penalties and sanctions can be imposed. Professionals, lawyers and accountants are under the obligation to issue this opinion letter, otherwise they may also be subject to significant sanctions.
On May 15th, 2007. See this statement in context.