I've purchased MPEG Player 3.0 and i found some things. Here is one of them. MPEG-4 video decoder included in the MPEG player package is Version 1.02 / Build 50214. When playing back a movie (DivX / DX50 , resolution 1280x720) in window mode, the aspect ration is 16:9 as it should be. When i switch to fullscreen, the movie is displayed with 4:3 aspect ratio.
When i use a different player than Elecard MPEG player 3.0 (e.g. WinAmp), it's even more worse: the movie is displayed always with an aspect ratio of 4:3. I see this effect on my computer monitor and on my TV set (where i can watch parallel).
With your MPEG-2 decoder i don't have problems like this.
Here is a link where you can legal download some HDTV movie trailers to reproduce what i wrote:http://www.divx.com/hd/
Edit: People reported, that the zip archive for "Skeleton Key" is defekt (so don't waste time downloading it), but the others are o.k.