I am trying to export 16:9 widescreen dv video, that I believe is 720 x 480, with a pixel aspect ratio of 1.2, to widescreen H.264 and/or Quicktime (without letterboxing, just as the footage was shot). However, in the export settings, whether I select a pixel aspect ratio of 1.2 or 1.0 (or .9, but that is way off), the video is stretched and looks terrible. If I export to MPEG2 with the 720 x 480 (1.2) settings, the video looks fine.

What is going on here?

Try 848 x 720, PAR of 1.

848 by 720? Could you elaborate on that?

I second that. 848 x 720 with a PAR of 1 did not work; it was more distorted.

If you are using h.264 I think you just need to do the math such that the ratio is 16:9. Or at least that's what I have been doing for YouTube and it works fine (I am using 640x360 there, for example). So my vote would be 852x480 with PAR of 1.

My guess is that Shaun made a typo and really meant 848x480.

Between 848 and 856 x 480 was a PAR of 1 is pretty close.

However, unlike when I export to H.264 from an HDV (MPEG2) source, when I play it back in a windows video player and full screen it, the letterboxing, that is, the black bars themselves, bounce up and down.

When I export in MPEG2, this does no occur; I get rock-solid letterboxing. This is a strange problem, but I'm assuming there is a very specific cause. Any ideas?

My guess is that Shaun made a typo and really meant 848x480.

Yup. My bad.

As well, 848 is divisible by 16 which is a requirement for some MPEG variants so I ALWAYS ensure that MPEG destined media is divisible by 16.

Nathan, what is the destination of your H264 video?

Computer playback? TV playback? It makes a difference...