There's a Halo Preview on PC.IGN.Com with first-hand impressions of Gearbox Software's upcoming PC port of Bungie's shooter. They discuss the similarities to the Xbox version, tweaks to make gameplay on the PC more workable, and what will be different, like the following bit on the weapons in the PC version:

There are even some new weapons here. Unfortunately since the Bungie and Gearbox guys had played the maps before they tended to snap up all the cool toys right away. Nevertheless we still managed to grab hold of the Fuel Rod Rifle and give that a try. This new weapon, used by the big squat Hunter dudes in Halo, has been made available in multiplayer. It fires a huge green energy blob that's the rough equivalent of the Marine rocket launcher. Very nice.