Not that its a great practice, it is allowed with some limitation. (With Plenum rated cable...)
800.53 (A)Plenum. Cables installed in ducts, plenums, and other spaces used for environmental air shall be Type CMP. Abandoned cables shall not be permitted to remain. Types CMP, CMR, CMG, CM, and CMX and communications wire installed in compliance with 300.22 shall be permitted. Listed plenum communications raceways shall be permitted to be installed in ducts and plenums as described in 300.22(B) and in other spaces used for environmental air as described in 300.22(C). Only Type CMP cable shall be permitted to be installed in these raceways.
Mark Heller "Well - I oughta....." -Jackie Gleason
Looks like I get smacked for "shooting from the lip." Apparently I was repeating something I'd heard, rather than actual code. Of course, you must forgive me, as I've never seen wiring in ducts (other than a short length to power something that was part of the duct system!). 300.21 and .22 seem to be the primary place where the NEC discusses wiring in ducts, with numerous other sections that boil down to "OK to run plenum rated wires in ducts." There ARE some other qualifications, so check it out.