The gemara (Avodah Zara 27) asks according to the opinion that women are excluded from the mitzvah of milah how is it that Tziporah performed a milah on her child en route back to Egypt? The gemara offers two answers: 1) she appointed a shliach to do the milah but did not do it herself; 2) she merely started the milah but Moshe finished it.
Anonymous in a comment elsewhere contrasts milah with shechita. Why is it that if a non-Jew begins the shechita process, even if a Jew finishes, the shechita is invalid (assuming yeshna l'shechita m'techila v'ad sof), but with respect to milah if a woman begins the process of milah it does not invalidate the mitzvah so long as finished properly by a man?
The Kli Chemdah (Shmos 2) raises this same question and answers with a pilpul based on the Rambam's view that all descendants of Keturah, Avraham's wife, are obligated in the mitzvah of milah as non-Jews. Tziporah was one of these descendents and therefore she was obligated to see that milah was done on her son -- the exclusion of women applies only to the obligation of milah that is incumbent upon the Jewish people. However, the Jewish people were given an added obligation of periya in addition to milah. This Tziporah could not fulfill and the job had to be completed by Moshe (this is a very rough summary of a complex series of l'shitasam arguments).
A more lomdish answer is that one can distinguish between the mitzvah of milah and the mitzvah of shechita. The mitzvah of shechita is defined by the act and process of cutting the animal's neck in a certain way, and therefore every step of that process must be done by someone who is obligated in the mitzvah. The mitzvah of milah, however, is not defined by the act of cutting of the orlah but rather is defined by the effect produced, the state of being nimol. So long as the completion of that effect involves someone obligated in the mitzvah there is no problem of other people taking part in the process along the way.
My son pointed out that one of his favorite achronim, R' Yosef Engel (in Shu"t Ben Poras Yosef), debates whether this point regarding milah is correct and suggests that it hinges on a dispute between the Rambam and Ra'avad. The Rambam (Hil Milah 1:2) writes that someone who has no milah is not obligated in the punishment of kareis until the moment of death when it is no longer possible to perform the mitzvah. The Ra'avad disagrees and says that every single moment the milah is delayed results in another chiyuv kareis. R' Yosef Engel suggests that the Ra'avad viewed the mitzvah of milah as being nimol, the effect of not having an orlah. Every moment that the person remains with an orlah is a violation of being in a state of not being nimol. The Rambam, however, viewed the mitzvah of milah as the process and act of cutting. Until the moment of death when that task can no longer be accomplished there is still the opportunity to avoid the penalty of kareis.