Observer Mode

Description
Observer Mode allows players of all levels, and locations (post-searing), to watch PvP matches amongst the top 100 ranked guilds. As of the Guild Wars World Championship 2006, championship matches of the concluded PvP season are also available for viewing. Matches that are currently underway are delayed by fifteen minutes to prevent cheating. By observing matches, players are able to study winning strategies employed by the top ranked guilds.

Observer Mode can be accessed while not in an explorable area or mission by pressing the appropriate hotkey. The default hotkey is 'B'.

Updates
From the initial update notes (Wintersday Update):

That's right, Observer Mode!

Now you can watch as top-ranked guilds battle it out in the Tombs and in rated guild matches. The upcoming Guild Wars Regional Playoffs and World Championship will also be available for viewing as they happen. The details are as follows:
 * Whenever a top 100 ranked guild plays in the Tombs or in a rated guild challenge, the game is automatically available for everyone to see. Other "special event" games will also be available for viewing.
 * Everything that observers see is delayed 15 minutes from the original game, to prevent cheating. Games become available for observation approximately 15 minutes after they start, and remain available until approximately 15 minutes after they finish.
 * While you are watching a game, you can chat with your fellow observers. There is no limit to the number of people who can simultaneously observe a game, but for purposes of chatting, observers are randomly assigned into groups of about 100 people.
 * In Observer Mode, the Map Area window displays the positions of all characters on the map. You can click on any character to jump there. Alternately, you can use Auto Camera mode to let the game pick the most interesting targets for you.
 * A new heads-up display automatically shows you the Health of teams that you are currently watching. If you prefer to use the Party window to view party status details and to select players to watch, you can easily switch the Party window back and forth between teams, or you may choose to view multiple Party windows and watch different teams in each window.

And the followup notes (see Game updates/20051222):

Improvements to Observer Mode:
 * You can now click on a player's name in the heads-up display to center the camera on that player.
 * The Map window now automatically repositions and centers itself on the selected character if that character moves outside of the visible area.
 * The list of observable games has better map thumbnails and sorts games by start time.