The more interesting question would be how many people THINK they are watching it in HD when they really aren't.
Probably 98% of 42" plasma TVs sold to date are not really HD, almost all sold are 480p. Usually they are 640x480, a few newer ones are 1024x768 which is at least close to 720p. Most (but not all) of the 50" plasmas can do lower resolution HD, i.e., 720p (they tend to be 1024x768, the widescreen ones are 1388x768 so they can display full 720p but not 1080i. LCDs tend to have higher resolutions but most are only 720p capable.
The picture may be better than standard TVs on a lot of these "HD" TVs people are buying, but very few are capable of displaying the full 1920x1080 that CBS broadcast.
Anyone who wonders why the uptake of HD has been so slow needs look no further than the fact there are well over a dozen different HD formats. With CRT sets it wasn't a huge problem, but with fixed pixel sets like plasmas, LCD or DLP its a pain in a ass. If its native resolution is set up for 720p, it displays a worse picture for 1080i input than if it had just been broadcast at 720p, and vice versa, and all the fancy $1500 DVDO scalers can't make the problem go away, despite what the salesdroids would like people to believe. I know a guy who "solved" the problem in his home theatre by getting TWO projectors, with some sort of $1000 gizmo that sends 720p content to the 720p projector and 480i/480p/1080i/1080p content to the 1080p projector!