yeah, doing the pow(x, 2); in the shadow mapping function doesn't do anyting
(it's the comparision between the linear filtering of the depth and the squared depth, that is used to calculate the linear interpolation for the shadows -> no hard edges and blurring is possible)

...since it looks like you know pretty much about vsm: does the depthmap has to be in range [0 - 1] or doesn't the range matter at all?