Okay, I'm going to give you all the gory details. Again, this is a D40U attached to an intel mac mini via DVI.
When set to 1920 x 1080 (interlace mode, the TV doesn't accept progressive), and overscan set to on, and in "dot by dot", the display is perfectly centered. With overscan off on and "dot by dot" the display is centered in the middle with a fairly substantial black border, I'd say 2-3 inches.
Now onto your question. When set to 1280 x 720 there is no "dot by dot" option, only "stretch" (I remember reading this in the manual now ... the view mode options change depending on the source). And again, overscan off has a border, but this time with overscan on it truly overscans ... about 2-3 inches of the display is cut off.
When set to 720 x 480 I now get four "view mode" options, which are: side bar, s. stretch, zoom, and stretch. And whether overscan is on or off, the display doesn't fit the screen perfectly in any view mode.
I'm getting a D90U sometime next week so I can test on that then, but I'd assume it to be similar. Also note, I think the source makes a big difference too, as when I was driving this thing with my powerbook (12" G4) I was getting different results.
For example, with the mac mini, when I first turn on the computer and the display in 1080i dot by dot, the display is shifted down a bit and cut off, by maybe 20 pixels or so. To fix this, I have to go into the display options, turn overscan off (which puts that large border around it) and then turn overscan back on, and then the display is perfectly centered (and looks great).
I don't remember having to do this in my (admittedly brief) powerbook tests. So your mileage may very depending on what is driving the signal.
Anyone else seen anything similar to what I see?