There arnt allot of hosts running this yet. One of the biggest hold backs is when you setup a new server, you must either supply it with a pre-generated world in the game install (which is bad because all customers will have the same 'randomly' generated world and might get cranky) or when you install it run it yourself on the server and tell it to make a new map.
This is because if you specify a world in the cmd line or cfg file, rathor than auto creating it like minecraft, Terraria throws a fatal error if it doesnt exist and dies, you can only manually create worlds via the server window.
I was excited about the release but the above caveat kinda killed my excitement.
Edit: I just spoke to one of the Terraria testers, He's gonna talk to the devs about above issue. See where that goes.
Edit: Just spoke to a developer, they are working to fix this and its high on the priority list.