[157.] If there is usually present a sufficient number of sacred ministers [priests and deacons] for the distribution of Holy Communion, extraordinary ministers of Holy Communion may not be appointed. Indeed, in such circumstances, those who may have already been appointed to this ministry should not exercise it....Addendum 4/14/2007:
[158.] Indeed, the extraordinary minister of Holy Communion may administer Communion only when the Priest and Deacon are lacking, when the Priest is prevented by weakness or advanced age or some other genuine reason, or when the number of faithful coming to Communion is so great that the very celebration of Mass would be unduly prolonged. This, however, is to be understood in such a way that a brief prolongation, considering the circumstances and culture of the place, is not at all a sufficient reason....
The "Norms for the Distribution and Reception of Holy Communion Under Both Kinds in the Dioceses of the United States of America" (approved by the USCCB on June 14, 2001) are posted on the USCCB website. What I find particularly noteworthy under "PART II -- Norms for the Distribution of Holy Communion Under Both Kinds," is the restrictive nature of these norms. That is, Communion under both species appears to be granted by way of indult, as an exception to the universal rule. To those who have studied the last four decades of liturgical changes, of course, this is will not come as a surprise. This is the way most of the post-conciliar changes have been introduced, from Communion in the hand to female altar servers.Update 4/16/07
Note the way in which this section is introduced under the heading: "When Communion Under Both Kinds May Be Given." Although the proceeding paragraph goes on to state that opportunities for offering Communion under both species have been significantly expanded in the revised Missal Romanum, the significant point is restrictive provisio under which the permission is extended. No carte blanche is offered here. The Norms go on to state the conditions under which the General Instruction of the Roman Missal (GIRM) permit Communion under both kinds:a. for priests who are not able to celebrate or concelebrateOf course, there are the standard equivocations that may be played for loopholes as well:
b. for the deacon and others who perform some role at Mass
c. for community members at their conventual Mass or what in some places is known as the "community" Mass, for seminarians, [and] for all who are on retreat or are participating in a spiritual or pastoral gatheringThe General Instruction [GIRM] then indicates that the diocesan Bishop may lay down norms for the distribution of Communion under both kinds for his own diocese, which must be observed. . . . The diocesan Bishop also has the faculty to allow Communion under both kinds, whenever it seems appropriate to the priest to whom charge of a given community has been entrusted as [its] own pastor, provided that the faithful have been well instructed and there is no danger of the profanation of the Sacrament or that the rite would be difficult to carry out on account of the number of participants or for some other reason.Yet whether even such equivocations can be abused to the tune of bevies of Extraordinary Ministers of Holy Communion that are regularly found in most AmChurch parish variety shows today (in violation of Redemptionis Sacramentum 157-159, cited above) is doubtful in view of such explicit statements as the following:In practice, the need to avoid obscuring the role of the priest and the deacon as the ordinary ministers of Holy Communion by an excessive use of extraordinary minister might in some circumstances constitute a reason either for limiting the distribution of Holy Communion under both species or for using intinction instead of distributing the Precious Blood from the chalice.
See the Congregation on Divine Worship's statement and commentary by Ralph here and here.