PBS offers quite a bit of HD programming (and SD widescreen programming). Some of PBS's programs are the most visually spectacular HD out there. The question is whether the local affiliate has the capibility to pass the digital signal. As PhilD mentioned NBC's the tonight show is broadcast in HD (Crossing Jordan as well, but NBC is far behind the other networks). In addition, most of CBS's primetime lineup and alot of ABC's primetime shows are also broadcast in HD. Fox broadcasts a handful of shows in EDTV (480p widescreen -- NOT high definition).
In respose to AVguyGeorge's post --
I have never seen a HD 4X3 program. It is not an ATSC specified resolution. A HD program could be sent as a 4X3 window within a 16X9 frame -- but I have never seen it done in the almost three years I've been watching HDTV (and this would only make sense if the program was derived from a telecined 4X3 film). What you were watching on HBO was most likely an upconverted 480i signal. They still have a fair amount of programming broadcast this way on their HD channel (sex in the city, Boxing, Six Feet Under, movies they feel don't deserve a HD transfer, etc.)
Hope this helps...