On the (two-layer) board I'm designing, I have a relatively large unused area. Instead of just pouring it with ground on both sides, I'm considering filling it with Vcc on one side and ground on the other, to create a small capacitance between the ground and Vcc. (I will still add adequate decoupling capacitance from regular capacitors, of course.)
The board is not exactly high speed (16 MHz microcontroller, doing only digital IO). And I'd be hard pressed to produced even 1 nF of capacitance from the available board area, I think. So you could argue that this extra capacitance is not doing much of a difference. But is there any reason why it might actually be a bad idea and should be avoided?