From the GIRM:
http://www.usccb.org/liturgy/current/chapter4.shtml
Quote:
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136. The priest, standing at the chair or at the ambo itself or, when appropriate, in another suitable place, gives the homily. When the homily is completed, a period of silence may be observed.
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(italics mine)
Apparently that priest finds the nave a suitable place for his homily.