A comment in the "horse barn" thread has stirred this question.
Can you run NM in conduit for any significant distance. My understanding of 336-6 (and now 334-15) is that conduit protection is used in short runs for specific hazards.
Because NM is a cable assembly and not just conductors, running it in conduit is "mixing wiring methods", and you're subject to overheating.

This has been cleared up in the 2005 NEC.

It is specifically allowed in the .22 sections of the conduit articles if the particular cable article doesn't prohibit it.

Roger

I don't see how heating is considered an issue at all, code wise. You derate the same in 1/2" conduit as you do in 4" for the same conductor fill. A piece of Romex in "free air" EMT will certainly run cooler than it would buried in insulation in an attic.

the .22 additions only point out rules that always existed. Basically you use the widest dimension of an oval cable as the diameter when computing fill. That is because when you get a twist in it the widest dimension becomes the diameter.

Romex is listed as a 60 degree assembly.
The THHN wire inside romex is listed as a 90 degree wire.

By putting romex in pipe, you are throwing away 30 degrees of de-rating, before you do anything else.

Also, the inside of conduit can sometimes be considered a wet location; romex is not allowed in wet locations.