NOtE: I've just noticed that I've unwittingly copied someone else's username. Apologies to *NSXFTW*; I should've figured that would've been taken. I've submitted a request to have my username changed.
Aaaaanywho...
It's actually not the first vehicle I've noticed it in, but in the NSX it's particularly strange because the engine's behind the cabin and I've always attributed it to engine compartment fumes sneaking in through the cabin air intake vents below the windshield. I'm pretty much positive that it's coming in through the air ducts, since it's stronger right at the vents when they're blowing, so I'm suspecting again possibly heater core? Maybe some coolant is evaporating into the cabin airstream? Did a search and looks like no one else ever reported a similar issue. It's quite concerning, since I've noticed that when it's particularly heavy, I get really drowsy... which does not mix well with driving. I'll check the harness plug, etc, and report what I find...