For some of us trying to sharpen our PCB layout skills, the right way to handle digital/analog ground planes can be a bit of a confusing topic. Eric Archer points out this straightforward article on the subject from Mr. Henry Ott

Some people suggest splitting the ground plane in order to isolate the digital ground currents from the analog ground currents.  Although the split plane approach can be made to work, it has many potential problems especially in large complicated systems.  Can you list some of these problems?  One of the major ones is that you can not route a trace over the split in the plane […] 
It is always better to have only a single reference plane for a system.

Hmm – ya learn somethin’ new every day. Read more on the Grounding of Mixed Signal PCBs.