When you consider that:
1) The 500M is significantly less money then the 600M, $1000+;
2) Reportedly the 500M includes some features that were destined for the 10g that didn't make in to the 600M;
3) D-Nice has made public ISF calibration settings that can apparently bring anyone's 500M close to calibrated if they follow his break-in routine.
4) There's a bit of a feeding frenzy as 50" plasmas on the market are dirt cheap and most stores are closing out the 50" models before they'll eventually move on to the 60" - which address a higher end market.
I imagine quite a few people were swayed to the 500M in spite of those of us who warn that TVs tend to shrink in a room and that 60" is great from 8' to 12'. It would be nice if HiFiFun would stop stating his opinion as fact, but I do agree with it.
On the bright side, for all those reasons, the 500M may actually see an increase in resale value once the spigot on new 500M's is finally cut off up until the point that something perceived as clearly better hits the market.