I think the answer is in 300.22(C)ex
Exception: This section shall not apply to the joist or stud spaces of dwelling units where the wiring passes through such spaces perpendicular to the long dimension of such spaces.
When I first read George's note I was thinking about wires running down the stud bay <the riser ref in CL2R>.
If they simply cross the bay it is probably OK but YMMV with various AHJs
I have seen it go both ways in suspended ceilings and computer floors too.
All IBM interface and power cables after about 1994 were plenum rated (AKA "DP"), tan colored. The blue ones were riser rated and tolerated until we could swap them out but the grey (unrated) ones had to go.
In other than dwellings 725.61(A) would affect low voltage wire, no matter which direction it ran.
(A) Plenum. Cables installed in ducts, plenums, and other spaces used for environmental air shall be Type CL2P or CL3P ...
[This message has been edited by gfretwell (edited 10-31-2005).]