Hmmm,
Just to clarify a little:
In an aircraft with a good 'map' display (ie, an FMS-equipped aircraft), it is still the case that for an NDB approach, the primary tracking aid is the ADF.
If you go 'out of limits' on the ADF indications below MSA, you must go around. Yes, usually the best way to achieve a good track is by using the map and the tracking information on it, but the means by which you are navigating must be the ADF indications.
And yes, in the past year, I have done NDB approaches in 737s.
It's worth mentioning that the GPS inputs on the 737-NG fail reasonably often (for a typical line pilot, twice a year would not be unusual), and I have had a number of problems with map shift, and once lost the map altogether during a non-precision aproach, because of a technical failure.
If you don't think map shift or the FMC is a problem, do some research on the A320 Addis Ababa incident a short while ago...
Regarding light aircraft, if the GPS in a Boeing, or its map, can let you down, then do you really want to place all your faith in a couple of thousand pounds' worth of panel-mount kit?