If you escape into a dungeon to avoid being killed in world pvp ...
then you lost, much worse than if you had just been killed. Not only did you show awareness that you lacked the ability to win the fight, but you showed that you are so scared for your little cartoon character that you ran like a girl into an instance. You don't even get the honor of having fought a good fight.
If I was GM of a pvp guild, I'd have a policy of instantly kicking any member who was such a coward that he went fleeing into an instance. I hope some of the guilds on Icecrown do, in fact, have such a policy or that they decide to adopt one.