When selecting an sPvP match, a prompt will be displayed, asking whether to join a team or to be a spectator. Upon choosing to be a spectator, you will be able to select a combatant on either of the two teams. Selecting a combatant will cause you to follow that character; additionally, fixed viewpoint locations can be selected through camera icons on the compass.
Spectator Mode cannot be used by any player in rated matches that impact the leaderboard with the exception of Authorized Shoutcasters. These players are shoutcasters that are selected by ArenaNet for doing quality shoutcasting in normal PvP matches. When two highly rated teams are matched-up together, an authorized shoutcaster will receive an in-game notification with the option to join the rated match as a spectator with the intention of shoutcasting the match. This system is set up in order to guard against outside interference in these matches.
A list of the current authorized shoutcasters can be found on the forums.