The 1080i Modeline I'm using is the following:
ModeLine "ATSC-1080-60i" 74.25 1920 1960 2016 2200 1080 1082 1088 1125 Interlace
...which seemed to be the most generic 1080i 60Hz modeline listed here:
I did have to use:
Option "ModeValidation" "NoVertRefreshCheck"
...to get the nVidia driver to accept it. Also note that I'm using an interlaced display (Hitachi RP CRT) and had to enable deinterlacing in MythTV in order to prevent serious periodic tearing when playing interlaced content...just like many others here, due to interlacing issues that apparently may never get addressed.
In any case, I could be mistaken, but I thought that using a name like "1920x1080" for your custom modeline could get interpreted as a built-in mode and not use your custom modline at all.