Originally Posted by Steve Tack
Worst case, you could output in any video format the Xbox can play and dump it out in side-by-side (SBS) format. It would be half the horizontal resolution, but that method means you don't have to be concerned with strange formats and HDMI 1.4, etc. It probably means you'd have to manually switch your TV to side-by-side mode when watching it.
Beyond that, I don't know. If you're going to be doing a lot of that, it might be worth considering hooking up a PC for that. You can do strange stuff like take full res SBS content and have playback software send it as frame packed to the TV.
I was under the impression that when Call of Duty Black Ops first came out (it had a 3D enabled mode) that the Xbox could only output side by side. You had to enable it in the game, and then set your tv to side by side 3D mode to display it properly
But recently, the Xbox 360 was given an update that enabled full 1080p frame packed 3D (some games are already utilizing frame packed 3D such as Batman Arkham City)