Following the very successful launch of the Russian localization, the vast majority of Russian-speaking players have migrated over to the new Russian-language realms. Four English-language PvP realms in particular were largely populated by Russian-speaking players, so these realms now have low remaining populations. As such, we've decided to offer free character migration off these realms to join larger English-language communities. Once the migrations are complete, these realms will be brought down and prepped for the Wrath of the Lich King launch.
More information can be found on the forums.