I fiddled with different television modes (1080i, 1080p, 720p) and with different screen resolutions (1920x1080, 1024x768) but no matter what the effect was the same. Puzzlingly, the NVIDIA screen size adjustment tool matched up perfectly with the edges of the physical display! But the Windows desktop still extended past the edges.
One would think that the all-digital connection would be easier to get right than the DVI-to-VGA connection I've been using.
I've seen lots of complaints on message boards about problems with the set (NVIDIA, DVI, HDMI, television) but not much concrete help. Tomorrow I may try taking my Dell monitor downstairs to try--- I've been using DVI-digital successfully for a while now. So that should at least tell me whether the output or the television is screwed up.