Ul'dah... Milvaneth Sacarium?

[Neasa]

If other people are still seeing their npcs, its likely just you hitting the cap on characters displayed and so the npcs are unable to load. It has happened to me on occasion as well, though normally for only a single npc at a time. It happened rarely in FFXI too. The way to fix it is generally to walk away really far, so that the models that normally would be there would unload. then just walk back and they should hopefully reload properly for you. About 50-100 yalms or so does the trick normally.

Likely you're seeing it due to the sheer amount of enemies in the city atm, which is pushing up the cap on models displayed.

This would make sense... If it didn't include Gridania when no one else was even in town, or moments in Ul'dah where there's no enemies up. :x
The NPCs are gone.