I didn't have to hold down anything, I just told the Xbox I had a 16x9 TV in the setup, and turned on all the modes it could do. The games take care of the rest.
Dvlos said:Good "widescreen" games are far and few between, I'd say check out Soul Caliber (not true widescreen but 720p), definately check out Ninja Gaiden, best looking Xbox HDTV game out there. Enter the Matrix is 1080i but the game is not that good, but you can give the game a try...
Halo 2 will be 480p unknown if it will be true widesceen, I hope so.
To the poster who was talking about the side bars, there is a way to make the Xbox boot into 480p mode, by holding (I think I could be wrong, Google this or check Xbox's site to be sure) L+R and X, then go into your settings and change it to widescreen, that will make all games default to widescreen, those that are "true" widescreen wil detect the setting and set itself accordingly.