Here are the relevant documents:

http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html
http://travel.state.gov/law/citizenship/citizenship_778.html

Specifically:

Quote:
In light of the administrative premise discussed above, a person who:

(1) is naturalized in a foreign country;

(2) takes a routine oath of allegiance or

(3) accepts non-policy level employment with a foreign government

and in so doing wishes to retain U.S. citizenship need not submit prior to the commission of a potentially expatriating act a statement or evidence of his or her intent to retain U.S. citizenship since such an intent will be presumed.

When, as the result of an individual's inquiry or an individual's application for registration or a passport it comes to the attention of a U.S. consular officer that a U.S. citizen has performed an act made potentially expatriating by Sections 349(a)(1), 349(a)(2), 349(a)(3) or 349(a)(4), the consular officer will simply ask the applicant if there was intent to relinquish U.S. citizenship when performing the act. If the answer is no, the consular officer will certify that it was not the person's intent to relinquish U.S. citizenship and, consequently, find that the person has retained U.S. citizenship.
_________________________
Bitt Faulk