It's funny that the topic posted by the OP was not about crop densities/resolution vs full frame, but it was comparing the upcoming Nikon 36MP full frame sensor to the upcoming Canon 18MP full frame sensor.
I say interesting, because the Nikon sensor is not "pushing" things as far as pixel density goes. Right now, the Canon 7D sensor has the equivalent density to what would be a 46MP full frame sensor. If Canon manages to "get it right" with the 18MP crop sensor, then it makes sense that in time a 46MP full frame sensor would follow.
So, people just need to bear in mind that both companies are putting IQ before resolution for their full frame cameras, and that neither of these full frame sensors are "pushing" any reolution limits. So instead of imagining a 36MP full frame sensor, look at the 7D and then imagine a 46MP full frame sensor!
As far as to how "real" more megapixels can be, well, pay a visit to a local mall/shopping center, where they put large promotional posters up at the windows where people will stand close and check them out. It's a fun exercise to look at one of those posters and realize that the ones that have a sharpness/crispness that just stands out were in all likelihood shot with an MF or LF camera, whether digital or film. Can our digital DSLRs match that output quality?
Well, I'd say we are progressing toward that point...but, I'd still love an MF digital!