IGN's Gear site has reviewed Lik-Sang's SmartJoy Frag, an adapter that lets you use standard PS/2 mice and keyboards with Xbox games. Here's what they had to say about using it to play Halo:

Halo just works damn fine with the SmartJoy FRAG. Movement is as smooth as butter and being able to spin around or look anywhere quickly is a godsend. Our only complaint with this is that the vertical movement is slower than the horizontal, but that's not too big a deal. Hopefully Halo 2 has options for both.

Editor's Note: Not sure why, with the Xbox's input being based on USB, Lik-Sang has chosen to use PS/2 ports instead. It means that you'll need to use USB to PS/2 adapters if you have a USB keyboard or mouse. Also, be aware: although IGN speculates the device should work with Halo 2 as well, there's no solid evidence of this, and it's entirely possible that Bungie may have made, or may make, changes to Halo 2's input code to negate any advantage that the SmartJoy Frag might give to keyboard and mouse players.