Rendering video with pixel aspect ratio 0.9091 (in premiere CS5.5)
If you're exporting directly from Premiere, then select the MPEG2-DVD NTSC preset in File -> Export -> Media, as shown in this image. 

Also, it looks like MP4Box will allow you to change the pixel aspect ratio flag in a .mp4 without re-encode.

FYI: 720x480 with PAR 0.9091 @ 29.97fps is the NTSC DV standard resulting in a 4:3 display aspect ratio. Similarly, 720x576 with PAR 1.0667* @ 25fps is the PAL DV standard.

*The actual PAR is supposed to be 59/54, but many apps use 16/15. See here if you're interested.