I've seen valid arguments for both, but it totally depends on what you have on the board,
and how you're connecting entry points to your ground plane.
In my own opinion, dual ground planes aren't necessary. Unless you're trying to save on the
cost of etching boards.
I've had over 40 boards FCC certified with only a single ground plane on the lower layer.
Edit: What AL said is good info. If you DO include a ground plane on both sides of your board
(make sure you have only 1 entry point from your power supply input) to both planes.
I.E. the ground from your power supply input connects to the entry point of the plane. There
should be NO ground traces to anywhere else on your PCB. Each IC should have a very short
trace directly to a "via" that connects directly to your ground plane. A 4-spoke thermal connection
point like the one in the attached graphic works extremely well - with single or dual planes.
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