you can become a canadian citizen as long as you get it before you swear an oath to be a us citizen. After the oath ceremony if you intentionally secure a citizen of another country you will "voluntarily " loose your U.S citizenship . see below:
The Department of State is responsible for determining the citizenship status of a person located outside the United States or in connection with the application for a U.S. passport while in the United States.
POTENTIALLY EXPATRIATING ACTS
Section 349 of the Immigration and Nationality Act (8 U.S.C. 1481), as amended, states that U.S. citizens are subject to loss of citizenship if they perform certain specified acts voluntarily and with the intention to relinquish U.S. citizenship. Briefly stated, these acts include:
obtaining naturalization in a foreign state (Sec. 349 (a) (1) INA);
1.taking an oath, affirmation or other formal declaration to a foreign state or its political subdivisions (Sec. 349 (a) (2) INA);
2.entering or serving in the armed forces of a foreign state engaged in hostilities against the U.S. or serving as a commissioned or non-commissioned officer in the armed forces of a foreign state (Sec. 349 (a) (3) INA);
3. accepting employment with a foreign government if (a) one has the nationality of that foreign state or (b) an oath or declaration of allegiance is required in accepting the position (Sec. 349 (a) (4) INA);
4.formally renouncing U.S. citizenship before a U.S. diplomatic or consular officer outside the United States (sec. 349 (a) (5) INA);
5. formally renouncing U.S. citizenship within the U.S. (but only under strict, narrow statutory conditions) (Sec. 349 (a) (6) INA);
conviction for an act of treason (Sec. 349 (a) (7) INA).
http://travel.state.gov/law/citizenship/citizenship_778.html
my 2 cents........