The latest edition of The Hookup, an audio-video and gaming column at Penny Arcade, talks about the impact of High Definition on gaming in general and Xbox Live in particular. Halo, of course, gets mentioned:

Fortunately for Microsoft, players of their best-selling shooters like Halo and Halo 2 might not suffer as much because of the limited dimensions of the in-game world. Shooters like Halo are fairly compact and there is very little need for 'fog' to give a believable feeling of space to the environment. Target recognition is largely based on color in this particular multiplayer environment. But for game environments that are vast, the best way to suspend our disbelief is for the game developer to adhere to the rules of our own natural world. In our world, colors appear to be 'washed out' at greater distances, and the shapes and details of an object become much more important for target recognition. Not being able to rely on color differences alone may tip the balance against a player with a low resolution screen. Developers may counter this by making objects in the field stand out with even greater contrast and brighter colors, to help the low-def customer. This still works in the favor of the HDTV owneramazing color reproduction is one of the hallmarks of the new High Definition video delivery standard.

The column is, indeed, quite a bit longer than this meaty quote; but it's well worth the read. Basically, the author sees the yawning technological gap between traditional and high-def video gear as being so wide that it will constitute a serious competitive advantage for the better-equipped players, much as high-framerate video cards do for PC gamers, but perhaps even more so.