connecting digital and analog grounds
All of the back-and-forth discussion above misses the real issue. It doesn't make any difference whether or not you use one continuous ground plane, a split plane, or two independent planes.
What you are really trying to do is keep the return currents for analog and digital separated. You don't want a digital signal to share a return path with an analog signal. If the two share the same path, you will modulate one with the other thus causing 'noise'.
Keeping the two return paths apart is as simple as routing the signal traces adjacent to the ground plane in such a way as to keep them separated. The return side of the signal loop will follow the path of least inductance (which in most cases is also the path of least impedance). So you can assume with a fairly high degree of accuracy that the ground current for your trace follows like a mirror image on the plane directly below the signal trace. Most AD and DA converters are bonded out in such a way that the analog and digital signals are on separate sides of the package. This makes it easy to run the signal traces away from one another, and use just one solid ground plane for reference.
Keep in mind that you have to look for physical obstructions, such as blind via holes and slots in the plane, that will force the return current to flow around them. Such physical obstructions make the return side of the loop larger since the return signal can't follow directly below the trace.
People tend to think that there is some sort of magic about using separate analog and digital planes. It is only the lazy man's way of ensuring that the two types of return current (analog and digital) don't mix. It is physically impossible for a logic level digital signal return path on a ground plane to infulence in any way the analog signal return path that is an inch away. In general, if the two types of signals are kept greater than 3 or 4 trace widths apart, the coupling between the two will be at least -50db.
The bottom line - do a careful layout. Keep analog and digital signal traces separated by as far as possible. Use a solid ground plane, and route the critical signals as directly as possible.
Remember that there is no magic or mystic way that analog and digital signals will find each other on a ground plane. Simply sharing the same plane cannot in any way cause mutual interferrence. In order for one type of signal to cause a problem with the other, the two signal loops have to be mixed physically and/or electronically. You control the fields around the traces with the size and location of the traces in your layout. Those fields are the only way signals can interact.