This has been asked on the Wizards boards, the answer Keith gave was...
There are 2 places you could put a "ghostwalk city". Either as a section of the mournlands (a region devastated by uncontroled "war-magic") or in a Necromantic Manifest Zone (a place with direct contact to the "plane of the dead").
He also added that he was concerned that a "ghostwalk city" would significantly change the "balance of power" of the setting. That it would be a focal point of many groups (those for and against necromancy).
Personally I believe that if "all the world's dead" traveled through that city, then it would be too powerful. If only those who died within 10 miles (for example) were affected then it might fit into the rest of the setting better. (Alternately, you could have the dead drawn in based on level. Any 1st level characters who die within a mile, any 5th level characters who die within 5 miles, ect.)
