Go through this thread
and you'll find a combination of a modeline at twice the frequency that SHOULD be required for 1080i and settings that stop the driver from rejecting it.
Once you have 1080i working, you should be able to use similar settings to get a native resolution modeline working.
I think the problem here is that nVidia, so far, has been trying to treat anything connected by DVI as a DFP. If we were talking about the average flat-panel desktop monitor, the assumptions that have been made are valid. However, a DVI-connected TELEVISION is a different critter all together and should be treated as such.