The Weekly Update over on Bungie makes a mention of Vista-only content:

Halo 2 Xbox players will continue to play against Xbox and Xbox 360 players, and Halo 2 PC players will play against other PC players. Although we will support other control methods, including joystick vs. mouse, the player will always have the choice, so if he feels confident he can beat a mouse-player online, that's his prerogative. We won't be forcing him. A game like Shadowrun was built from the ground up to run on both platforms with finely balanced cross-platform gameplay. We don't have that luxury, and instead intend to make the Vista version of Halo 2 the best it can be. Add to that the fact that we will have new maps built specifically for the Vista version and a map editor, and cross-platform matchmaking becomes even more complex and less realistic.

This special treatment is causing some rumblings from the peanut gallery. Some are miffed about the exclusive content and not being allowed to play with Xbox users, others about waiting two years for Halo 2 to show up on the PC at all. Still more object to the sheer lack of oomph in using such an aged game to launch the new OS. I am sympathetic; there's a lot to bitch about here. Eliza Gauger

Weekly Update 5/26/05 [Bungie]