I believe she is not. Plus, I may not know Fed Cup rules to a detail, but I don't think she would be able to play for Russia if she'd have dual citizenship.The U.S. does not allow dual citizenship. To become an American citizen, you have to renounce all other nations' citizenships.
Are you sure Sharapova is an American citizen?
I was reading about Davydenko applying for a citizenship in Austria (in addition to his russian one) and they said he won't be able to play for Russia in Davis Cup after that.
Maria acts and speaks like an average american teenager. A lack of proper education shows too, - did she even had time to finish high school properly?
Her russian is all right for somebody who grew up outside of Russia and not reading russian literature or watching russian TV etc.
AS far as USA not allowing dual citizenships, it is true, but i think it is just formality. There is no procedure where you suppose to bring in all your foreign passports and hand them over to INS officer
A lot of new american citizens still keep their foreign passports - it makes it easier to travel etc. Plus, believe you or not, it costs money to officially give up russian citizenship through the consulate