when pursuing Clinton:
This is what John Dean has to say on the issue when this came up during the initial Plame Scandal:
A divided U.S. Court of Appeals for the Eighth Circuit agreed with Starr. The court held that a grand jury was entitled to the information. It also held that government officials -- even when serving as attorneys -- had a special obligation to provide incriminating information in their possession.
In the second case, In re Lindsey, Deputy White House Counsel Bruce Lindsey refused to testify about his knowledge of President Clinton's relationship to Monica Lewinsky, based on attorney-client privilege. Starr sought to compel Lindsey's testimony, and he won again.
This time, Starr persuaded the U.S. Court of Appeals for the District of Columbia Circuit to follow the Eighth Circuit. The court ruled that exposure of wrongdoing by government lawyers fostered democracy, as "openness in government has always been thought crucial to ensuring that the people remain in control of their government."
Based on these precedents, President Bush has almost certainly been told that the only way he can discuss his potential testimony with a lawyer is by hiring one outside the government.
http://writ.news.findlaw.com/dean/20040604.html Edited to add: Of course, the emails using the RNC domains take away from the executive privilege claim as well.