Several of Apple's QuickTime H.264 trailers are encoded with 5.1-channel AAC audio. As you may have read in the Xbox 360 H.264 playback article, these files will not play on the Xbox 360, because it only supports stereo 2.0 AAC audio.
In order to convert these files quickly to work on the Xbox 360, one can use either QuickTime Pro ($) or else QTAmateur (free). The latter is a freeware Mac OS X program which simply exposes the "Pro" functionality already built into QuickTime.
To convert the files:
1) Select File --> Export --> Movie to MPEG-4.
2) Click the Options button to get the MPEG-4 Export Settings screen.
3) Select MP4 (not MP4 (ISMA)) for the file format.
4) Click on the Video tab, and select Pass through.
5) Click on the Audio tab, and select AAC-LC (Music), 160 Kbps (or whatever), and Stereo.
Because the video is not re-encoded, the conversion is lightning fast. The resultant exported file will use the .MP4 container format instead of .MOV, but will continue to play fine in QuickTime, and now will also play on the Xbox 360. This procedure will work with files up to 4 GB. Larger files should be split into smaller segments before export.