Nope, not a dumb question! I think you may have stumped a few people. Used to use it a lot for control work and it was always dual rated. Looking at the properties it appears that the MTW has a tougher insulation.
MTW = Flame Retardant / Moisture, Heat, and Oil resistant / Thermoplastic
THHN = Flame Retardant / Heat, resistant / Thermoplastic
A house is not a machine tool, but does the MTW exceed THHN or THWN specs?
I think it's an AHJ call it seems to be a grey area. This is the chance homeowners take when they do there own wiring.
Did a little research though, NFPA 79 Ampacities table: http://www.aiwc.com/catalogsection/catalogpdfs/s23web.pdf
[This message has been edited by wirewiz (edited 07-31-2002).]