The Official AVS HDTV Programming Synopsis is a topic on the AVS Forum with good, up-to-date info on what's on HDTV. Not specific programs, mind you, but broadcast, cable, and satellite networks and what they carry.
As for specific HDTV program schedules, TitanTV is a website that offers them. You enter your ZIP code, it shows you the relevant schedule in your area. Unfortunately, it claims the schedule for my local HD PBS outlet, listed as WMPT-DT (the DT stands for "digital television"), is "unavailable."
You can personalize TitanTV. I did that such that it (thinks it) knows exactly what cable and DirecTV channels I subscribe to. Then I selected the "All Hi-Def" category atop the listings. That gave me a grid with just the HD channels (on Comcast cable) ... except that channel 200, the Comcast SportsNet HD feed is missing. Alas! (But MPTHD, alias WMPT-DT, is there!)
Another quick way to see what's on HDTV, on the various broadcast, cable, and satellite networks, is to visit HDTV Galaxy. From Monday through Sunday, a week's worth of listings — series, movies, sports, etc. — are stacked on one web page with clickable anchors to get you to the day you want. One of the nice things about this listing is that only HD programming appears in it, making it easy to filter out non-HD fare from your viewing plan. No regional sports nets or local stations are listed. (Alert: Harry Potter and the Prisoner of Azkaban premieres on HBO in 1080i this coming Saturday, 6/4/05, at 8:00 PM EDT.)