3dplot of modulus of complex function

edit

This is may not be a good answer explaining why is the Symbolic Ring not working here, but at least a workaround: forget about symbolic functions and use Python lambda functions instead:

sage: plot3d(lambda x,y: abs(x+I*y), (x,-5,5), (y,-5,5))

or even:

sage: plot3d(lambda x,y: abs(x+I*y), (x,-5,5), (y,-5,5), adaptive=True)