I've been working for a few months now to get 720p output from my system to my LG dlp 52" tv. It would never work, and 1080i worked with like half the screen, like the left and right met in the middle of the screen. Finally i found this website:
http://www.linuxis.us/linux/media/howto/linux-htpc/index.html

and use the modelines here:
DTV-MovieModes
ModeLine "ATSC-480-71.93p" 28.699 640 664 736 760 480 484 492 525
ModeLine "ATSC-480-72p" 28.728 640 664 736 760 480 484 492 525
ModeLine "ATSC-480-119.88p" 47.832 640 664 736 760 480 484 492 525
ModeLine "ATSC-480-120p" 47.88 640 664 736 760 480 484 492 525
ModeLine "ATSC-720-71.93p" 89.011 1280 1296 1360 1650 720 722 728 750
ModeLine "ATSC-720-72p" 89.1 1280 1296 1360 1650 720 722 728 750
ModeLine "ATSC-720-119.88p" 148.351 1280 1296 1360 1650 720 722 728 750
ModeLine "ATSC-720-120p" 148.5 1280 1296 1360 1650 720 722 728 750
ModeLine "ATSC-1080-71.93i" 1920 1960 2016 2200 1080 1082 1088 1125 Interlace
ModeLine "ATSC-1080-72i" 1920 1960 2016 2200 1080 1082 1088 1125 Interlace
ModeLine "ATSC-1080-71.93p" 178.022 1920 1960 2016 2200 1080 1082 1088 1125
ModeLine "ATSC-1080-72p" 178.2 1920 1960 2016 2200 1080 1082 1088 1125

and the ATSC-720-71.93 and 72p worked for me (ok don't know exactly which but one of them does).

Also i took this suggestion:



My tv manual said that the pc input couldn't go above the 1024x768 resolution, and i noticed that it just wouldn't accept the 720p resolutions or most of the 1080i reolutions, it just kept trying to probe. So i didn't try the new resolutions i got with DPMS, i just disabled it and went so it worked.

Just wanted to share.

I also have a 52" LG DLP, and the RGB input on the back of the television has two modes, one is RGB-PC and accepts a plethora of 'normal' PC resolutions. The other mode (the two are toggled between each other by hitting the RGB button on the remote while already in an RGB mode) is RGB-TV or RGB-Digital or something, and it supports (according to the manual) 1080i, 720p and 480p.

I'm not sure, but it looks like you've found some additional modelines that your television supports. Search this forum for "modeline ATSC 720" (all of the words) and you'll find a post that talks about ATSC-720-59.94p and ATSC-720-60p which you havent referenced, and which work for my LG DLP television. I think technically your 720 and 1080 modelines are not set to valid resolutions, because there are no ATSC 720 resolutions that have refresh rates in the 120Hz range that I've ever seen listed.

Sorry it too you such an extended period of time to get your resolutions working though. But congrats on your success.

Jason