I have a design that has a 900 MHz receiver and a wifi module on the same PCB. It is a four layer design. My thought is to define three grounds, analog1, analog2, and digital. Each section of the board will have it's own ground plane, both the internal ground and the ground pours.
Each ground plane/section will be connected to the adjacent ground through a single ferrite bead.
Most people prefer a continuous ground plane in this situation. Otherwise, your multiple ground planes form an antenna and may considerably increase the receiver noise level.