useful link http://stpetersburg.usconsulate.gov/...formation.html

full of useful warnings :-)

Russian immigration and visa laws have been recently changed, and, reportedly more changes are being contemplated. The implementation of these laws has not always been transparent or predictable. In addition, Russian Immigration officials at times implement the laws and regulations governing entry and exit inconsistently, especially in remote areas.

U.S. citizens who have at one time held Russian citizenship are often required to renounce Russian citizenship before applying for a Russian visa in their U.S. passport. Unless a former Russian citizen has formally renounced his or her Russian citizenship through a Russian Embassy or Consulate, he or she always risks being considered a Russian citizen and not allowed to depart on any travel document except a Russian passport. This can also interfere with access to U.S. consular services in case of an emergency. This risk is greatly diminished if the traveler enters Russia on a U.S. passport and Russian visa.