I recently found that videos wouldn't play properly on my netbook. The screen was just black.
After spending hours installing codecs, I finally noticed that the nautilus thumbnailer was dealing fine with them.
It turns out that the problem is that I've currently got my little Dell connected to a second screen.
Changing the display mode to be one screen only cures the problem (It doesn't matter whether it's the laptop screen or the external monitor, as long as one of them's switched off).
Using mplayer, you can also make it work by changing the video driver
$ mplayer -vo gl film.mov
Uses the gl driver instead of the default xv, which appears to work better.
the x11 driver also works, but you can't resize the picture.
Just for reference, my netbook is a Dell Mini 10v, running Ubuntu 9.10.