Sealed Deck is a game type that has been used at past Penny Arcade Expo showings of Guild Wars, and it has become popular enough to warrant being added into the game. The main drive behind changing up Team Arena and Hero Battles are due to the waning number of players that participate in those formats, as well as some very brutal flaws in the Hero Battles format.
Beyond the addition of Sealed Deck, Linsey also details some other changes to PvP, such as the addition of henchmen that have builds designated by the community, temporary downtime for the Xunlai Tournament House, and many other tasty tidbits. If you're looking for the full info, check out her post over on the Guild Wars wiki.
