Offered in a friendly collegial tone...
I'm not doubting your expertise, but I am sure noise margin is widely defined in terms of voltage:
http://encyclopedia2.thefreedictionary.com/noise+margin - it's just implicit in the name.
Noise increases like the root of the bandwidth and is measured in volts. There is a another unit
noise power, that is proportional to the bandwidth and is measured in watts. Signal is defined as volts. So when I say "signal" I mean in volts. dBm is only relevant to the power measurements column, where you need a baseline to compare to. For noise margin, there is no need for a baseline, since you are comparing the signal to the noise.
Basically, it does not matter really ... as long as you understand whether you are comparing voltage or power. You can get the NM in dB by comparing the noise to the signal (in volts/volts) or by comparing the signal power to the noise power (watts/watts). You will get the same value in dB, as long as you use the proper multiplier (10 or 20) appropriate to the inputs.