Quote from the last interview where Yoshida mentioned Zone congestion and the possible solutions they are considering.

Aiming for 5000 players per server, 1000 players in one zone lagless
Two approaches to zone congestion

Display a message "too many players in zone, entry denied"
Spawn instances of a zone to allow for more players

My opinion:

Just locking the zone is of course the cheaper solution dev cost wise and 1000 players in 1 zone is not something that happens often.
But this would cause problems if popular events start happening or something like last event in 1.0 so its not the best solution.

Temporal instances would need an overflow server and probably cost alot more time and money. Obviously the better solutuion (for us).

I would suggest going with the overflow server, since at release a lock up could seriously anger some people and you know how cramped it can get first.
Yoshida probably wants a smooth start so he should take that into consideration.

What do you think?