With displacement maps Stonemason is definitely right.  Black is the null value and white is the highest value, it works more on a mask system than on a greyscale system.  All tones of grey between black and white have varying effects on the output depending on the closeness of tone to either end of the scale.  

The reason a flat 128,128,128 map when connected to the bump and the displacement appears to have no effect is simply because you are applying a flat plane which will effect all parts equally, thereby negating it's effect.  Further painting onto this map with varying shades of grey will giive an unholy mess as it then reads the 128,128.128 as a displaced area and off it goes.  Displacement in one ares has a knock-on effect in surrounding areas even if they are "masked" out by the black base.

Yes, I am currently trying to teach myself how to get the best from displacement mapping by the time honoured method of trying it and making a right mess but it is slowly coming together and the first thing I learnt was that only black will do for areas you do not want any displacement showing.

From my few experiments svdl I have found that displacement mapping is far more reliable and accurate than my bump maps and, no doubt because it deforms the actual mesh into that shape, it behaves more realisticlaly under any lighting than a bump map ever will.