Now - the GIRM specifically states:
St. Joan of Arc has a different lay "speaker" nearly every week.
66. The Homily should ordinarily be given by the priest celebrant himself. He
may entrust it to a concelebrating priest or occasionally, according to
circumstances, to the deacon, but never to a lay person. In particular cases and
for a just cause, the homily may even be given by a Bishop or a priest who is
present at the celebration but cannot concelebrate.
UPDATE: Gerald Augustinus over at The Cafeteria is Closed goes even further.