Well I run desktop in 1920x1080. I haven't changed anything with the SH3 graphic settings or used any graphics mod for widescreen, I don't run in windowed either. I just fire up SH3 and this is what I get.
http://img543.imageshack.us/img543/9674/lmez.jpg
Was playing around with a mod of mine, that's why the dials out of place.
I have full screen and perhaps a slight bit of stretch, but it's not really noticeable.
When I exit the game, I'm in desktop 1920x1080.
For some reason the screenshot displays at 1024x768. But I have a widescreen monitor and no borders.