Nielsen's latest numbers project that three-quarters of US TV households now have HDTV sets - a gain of 14% in the last year. More than 40% of US TVHH have and use multiple HDTV sets. But it's not just ownership - 61% of all prime time viewing is done on HDTVs.
Nielsen took a deeper look at its May numbers to take a deeper look at "true HDTV" viewing. Nielsen defines that as using an HDTV set connected to a HD set-top box that receives HDTV channels and programming (what - no over-the-air reception of HD broadcasting local stations?). Looking at 17 "HD" channels (5 English-language broadcast networks and 12 cable channels), they found that 29% of broadcast and 21% of cable channel prime-time viewing was "true HDTV" watching.
This might seem low, but the Nielsen definition and sample are very narrow - so the numbers should be treated as minimum levels rather than actual amount of HDTV viewing.
Source - Most Prime-Time Viewing on HDTV. Quality An Issue, MediaDailyNews