TbaMUD actually uses an unsigned SHORT int for the world index which allows for amount of rooms/mobs/obj that rumblke listed. Changing it to an unsigned INT will give you 4.2 million possible vnums. There are significant changes required to go that route though as many ingame variables are of the INT type and would be incompatible with an unsigned int.
In my MUD I went with just a plain INT which allows for 2.1 million vnums which is more then enough. The only changes necessary were in structs.h, and a couple save/load functions that natively used unsigned short for vnums.
65535 rooms is more then enough for hand written rooms but when using an overhead map for a continent, a 100x100 area is a decent sprint for a player and it takes up 10,000 rooms. Upping the limit allows for multiple continents, and written zones without concern for the possibke limitation. Plus, it's easier to organize IMO.
tbaMUD developer/programmer