National Electrical Code 2008
300.22 Wiring in Ducts, Plenums, and Other Air-Handling Spaces.
(B) Ducts or Plenums Used for Environmental Air. Only wiring methods consisting of Type MI cable, Type MC cable employing a smooth
or corrugated impervious metal sheath without an overall nonmetallic
covering, electrical metallic tubing, flexible metallic tubing,
intermediate metal conduit, or rigid metal conduit without an overall
nonmetallic covering shall be installed in ducts or plenums
specifically fabricated to transport environmental air. Flexible metal
conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to
connect physically adjustable equipment and devices permitted to be in
these ducts and plenum chambers. The connectors used with flexible
metal conduit shall effectively close any openings in the connection.
Equipment and devices shall be permitted within such ducts or plenum
chambers only if necessary for their direct action upon, or sensing
of, the contained air. Where equipment or devices are installed and
illumination is necessary to facilitate maintenance and repair,
enclosed gasketed-type luminaires shall be permitted.
Which means, NO. The only wiring allowed in ducts, is wiring that is "necessary for their direct action upon, or sensing of, the contained air.".
Mike Holt explains it well in this YouTube video.
If you read article 725, you'll see that it references back to section 300.22.