i dont believe the problem exists with NEF's
but i see it occur if i shoot jpeg ... then pull jpegs directly into photoshop CS, make changes and save to the jpeg. CS omits the "sensitivity" field when saving as it doesnt read that field. It only knows to look for "ISO"
From a website perspective, most web softwares like gallery, coppermine etc look at the "ISO" tag in exif when displaying exif info, so even if the "sensitivity" tag IS kept intact during post processing (lackthereof) .. most websites will ignore the tag. (though if you downloaded the master image and ran your own EXIF reader over it to see "all" exif tags, you would indeed see "sensitivity" )
two examples, both shot jpeg (dont ask.)
This one had the "isofix" tool run on it :
http://darb.net/select-photos/DSC_6908
this one didnt :
http://darb.net/select-photos/DSC_5888
(you notice under each image the website software dumps important exif tags, but the cityscape one has no "ISO" because i forgot to run the tool.)