How do you build a "wet room" style bathroom floor?
Assuming you are using tile as the floor I don't think there is too much to do.  You will need to determine the appropriate slope to the floor drain and put down a waterproof membrane (like Schluter Systems' Ditra http://www.schluter.com/2080.aspx) below the tile.

The IPC covers floor drains in section 412 but it does not specify a slope on the floor.  A shower floor is supposed to have 1/4" slope per foot (section 417.5.2) but I don't know if that implies the whole floor should be like that.