I'm a bit confused. Why do you want the video to fill the full screen?

When viewing a movie on an old-style 4:3 TV, there was two options.

1) The TV only shows the middle part of the movie. Sometimes cutting off a large part of the image to the left. Sometimes to the right. Showing just a part of the full image frame is called P&S - Pan and Scan.

2) The TV displays the full movie image, but must instead add black bars at top and bottom. This is called letter-box.

3) Some movies are shot on a 4x3 film but the camera has etched markings for the wider/lower cinema format. So all main action are within the cinema format while the film stores extra information at top/bottom that will not be visible at the cinema. Such film can be displayed on an old-style TV either using the full 4x3 film frame, or by letterboxing the part shown on cinema.

Anyway - when you download photos or movies on the net, they have different height/width formats depending on used camera or camera setting or their intended usage.

The majority of users wants a full-screen display to show all image information, which means that a mismatch in height/width proportions between screen and film/photo requires the display to have black bars either at top/bottom or left/right side.

Uniform zooming results in some part of the image not being visible. This can be look very good if a creative person have manually directed how to zoom - as in the pan-scan movies. Just have a media player application show the center part of the movie can give horrible results. A movie can have two people talking to each other and the player program only shows two noses at tle left and right side of the display.

Nonuniform scaling "fit-to-display" is not so fun either. Depending on the stretch direction, people will either be displayed as very fat or very skinny - unless they go to bed in which case the fat guy becomes tall and skinny and the skinny one becomes short and fat...

So once more - exactly why do you want your N900 to fill the full display when having a movie that has different height/width proportions than the phone display?