Effectively, Canola will stretch the video if opening an .avi file. The built-in media player will not open an xvid/divx file. By just changing general = mplayer to general = oms, my mp4 files are not stretched and play properly because they use the built-in media player instead of mplayer (which can't play .mp4 files any faster than 1 frame per second anyway).

All this is on a final N810/OS2008 released this week, no idea why it wouldn't work for some of you but works fine for me. I didn't, however, change the mplayer = mplayer line in my config file. To sum up, this is what I have in my config file :

audio = oms
mplayer = mplayer
general = oms