Question about t4i video resolution
It's because the SD resolution isn't 16:9. You either want HD or SD, and they're different aspect ratios. When I reduce the resolution to SD from HD, the video maintains its HD aspect ratio, but you have to go through the editing in HD before exporting to SD.
Where did you make this up? DVD movie resolution is generally 720x480 aka wide screen movies. There's no reason it can't be made an option in dSLRs for recording - or evenin P&S cameras.