[chmodd]

Depends on the definition of jumping. Besides paid character transfer (which is a "true" jump from realm to realm) a lot of activities in WoW take place in "instances" and players from different realms may participate in the same instance (PVP matches, dungeons, raids). A few weeks ago crossrealm zones were introduced, so normal questing areas can now span several realms too. It's kind of a hybrid between EVE's approach and classic UO's "shards as isolated universes" system.