Gremmi: As for media - it is correct that films are not usually in 16:9 aspect ratio anyway - however Blu-Rays (and DVDs) -are-, the source itself includes letterboxing, meaning there is no additional scaling needed, presuming the display is at 1080p. The display and output are perfectly matched.
With a 16:10 monitor at1680x1200, there would be scaling involved regardless.Either the GPU or monitor itself would scale the image to the correct ratio, meaning the output is different to the original source. 1080 horizontal pixels on a display designed for 1200 horizontal (though some monitors at this resolution do have displays that allow perfect pixel reproduction of a 1080 image by simply discarding the extra lines, it depends on how decent a monitor it is. Many cheap models don't discard the pixels and scale the image instead).
Even if the monitor does scale perfectly, you'll end up with visible letterboxes on top of letterboxes this way, which is quite distracting to watch.
Of course, the differences of scaling probably aren't detectable by the human eye, so this is a moot point. I just like the maths and figures involved in the discussion!
EDIT: Oh, and of course, this particular sidenote only applies to blu-rays and media with letterboxing already added. If you're watching an .avi or similar that has already had the letterboxes cropped off, you wouldn't get the double letterboxing effect.
Yeah I meant to say that originally. No seriously, I was a little lazy and didn't feel like writing all that. The people at blu-ray.com would kill me if they found out I cheaped out and tried to pull a fast one lol.