Voronoi diagram of point clusters?

Is there a way to compute the voronoi diagram of groups of points?

I.e. all points with the same label/value should be entirely contained by one voronoi segment. Computing the centroid of each group and using that for a regular voronoi diagram will not guarantee that points of the same group are contained within the same segment.

Calculating the convex hull of each group is also not satisfactory as it does not guarantee that either all segments are joined, or that segments do not overlap.

I am trying to derive approximate postal-code boundaries based on lat/long information for individual addresses.

The data itself is in postgis, but any language is suitable (extra points for solutions using sql, python or C++)...