Ichigo,
The 2407wfp for whatever reason disables the aspect ratio options in the menu when using the Xbox 360 over VGA @ 1080p
This is NOT a problem on the 2405fpw. 2405fpw users have reported that they are able to change between 1:1 and Fill.
For now, anyway, there's no benefit in running at 1080p unless you have the HD-DVD drive for the 360.
I've had the same problems with my 2407wfp and there is absolutely NO FIX for it. The only possible fix would be for them to include an Aspect Ratio option in the next Xbox Update, so that we would be able to select 16:10.
By the way, if you haven't noticed, there is also a problem with the video at 1080p. You will sometimes notice a sort of grid of black lines in different areas of the screen. The fix for this was found to be setting the Sharpness setting down to 30 in the screen's menu, but I've found that I have to turn this down to 30 EVERY single time I turn my Xbox on and switch to VGA.
So, moral of story ---- Dell sucks.
EDIT: I included a link to a Dell thread with lots of info and some pictures of the "black lines" and sharpness problem
Dell Thread