What is Live Anywhere?
Live Anywhere is the Xbox Live service that has been extended to the Windows Vista & mobile platforms. You can think of it in terms of the Xbox 360 Live guide blade on the Windows platform. That means you will be able to see and manage your Friends list, manage your Gamertag and account details, and access Live messaging services. As with most Live games you will be able to find your Friends no matter where they are and join them in your favorite games. In some games you will be able to play your friend when they are on a different device (Xbox 360, Windows Vista, Mobile), this is the cross-platform play that you have seen mentioned in titles like Shadowrun, although it should be noted that cross-platform play is not supported by Halo 2 for Windows Vista.

How many players will be able to play together on Windows Vista?
If it's not broken, don't fix it! We are going to have all of the original multiplayer maps and the map pack. These were all expertly tuned by the folks at Bungie to work for up to 16 players. We do have the addition of a Map Editor and may ship with a few new maps, we have chosen to make these 16 player max as well to keep the game experience as consistent as possible between the original maps and the new ones and to help keep groups together between games by not having huge variances in map sizes. This is especially important with Playlists in dedicated server sessions.